TL;DR short version
You have a reasonable list, though I would add a tarp or some other water-proof shelter that is quick and easy to set up and take down. This is to increase mobility or in case an emergency shelter is needed. Natural shelters are cool, but relying only on them slows you down a lot, I mean a really lot, and I'm still understating it. Just toss a tarp in your pack, even if it's a small one.
A knife, something for water that is cookable, something to start fire, a tarp. This does not include special items that depend on the location or duration; other than such specialty things, this list is it - all that's needed for short-term extreme-minimalist camping.
If longer than a few days, then some food too (for 2-3 days, identify and eat what you find). And depending on the climate, a sleeping bag.
Optional (but highly recommended): Water filter, rain coat, paracord.
Paracord only optional if plants very easy to use and good for lashing are abundant in the area.
Since you started with packs, I'll just mention quickly that I prefer the military-grade molle ones. Mine has taken much abuse, including getting rained into and being left with water inside of it for a while, and it looks beat up but has not failed me yet.
You have stated, combining your question and comments (you should edit your comments into your question), that you are speaking about the bare minimum, extremely rough, minimalist survival scenario.
For that scenario, technically you do not need anything. I have heard of a TV show about a guy who goes out with nothing but camera and medicine (not even clothes!) and makes everything on the spot needed to survive. I haven't seen it yet but I plan to watch that some day. However, that situation is a silly way to spend a vacation, even if you are into the super-minimal setting.
So we need to lay some ground rules to judge by; you have given us a great foundation for that. My answer will assume that...
you want to stay relatively comfortable
a. no freezing
b. no heat exhaustion (though I won't be addressing this much since I'm far enough north it's simply "sit under a tree" around here)
c. no feeling miserable because of your conditions (ex: getting rained on when you were not prepared)
d. no going without a reasonable amount of water for more than a day
e. no spending 100% of your effort simply subsisting by foraging for food and water all day
you want to do this with the minimal amount of help from modern civilization
you want to be mobile enough that you can pack up your few belongings and move when you want or need to, and to be able to continue as you were in the new location
a. I mentioned "or need to" to account for emergencies which require evacuation or avoiding natural events such as fires, tornadoes, etc.
some extra items for extra comfort can be brought as long as they are tiny enough and lightweight enough that they do not get in the way at all
you want to stay alive, and preferably not permanently injured in some way
a. you did make this sound optional in one of your comments, but I'm going to ignore that in my answer, otherwise the answer could be "Just go out with nothing. If you die, so what?" ;)
Going with that, you meet or exceed everything except some form of shelter that allows you to retain good mobility.
Technically, you can make almost anything you need, but remember the parts about comfort, subsisting, and mobility; that's what keeps us reasonable. So here's my food for thought...
Knife: Obvious. However, sometimes I carry a multi-tool - a great survival multi-tool will have at least a knife and tiny saw, and preferably a few other useful survival things; I've even seen some that have little hatchet heads on the other end of them.
Water: I'm right with you on the water container: 1 or 2 containers that can hold water and which can be cooked in. At least 1 of said containers should be able to be able to be closed and be water-tight, which yours are. Like you, I would bring these with some water pre-loaded. I would also suggest that at least 1 such container be one that you can reach into easily - no small neck like some canteens have - so that you can more easily cook larger things in it or clean it; this might mean the other container is more of a pot than a proper canteen.
Rationale: having a closable, water-tight container has obvious benefits, but having a container you can reach into and cook larger things in is for multiple other reasons. Yes, you can cook things skewered on sticks or placed on stones (flat stones work well for cooking on), but this is easier to clean, and you can boil larger foods as well.
(Optional) A water filter will put even more water sources easily at your use. I would get the smallest, lightest-weight one I could find. If no other water sources are present, a filter allows you to clean the water from mud or even to dig straight down where you're standing and get water from the muddy pit you dig, assuming the water table is high enough. This also keeps your canteens cleaner; you could boil muddy water in them, but that's not pleasant and leaves them dirty.
I have seen some articles and videos about making your own water filters by crushing charcoal you make from your fire. I tried that, and it did indeed clean the water, but the water flow in the one I made was sooo slow that it could not be considered a reasonable source of drinking water for any extended duration. Maybe if I tried it a few times I could get one to work right, but if you are spending times making and remaking water filters in the wild then you've dropped out of the criteria I mentioned above that I'm using for this answer. Note though: when testing the water filter I made, I mixed up some very dirty, muddy, nasty water to put into it, and what few drops of water came out the other side were pure, clean, clear looking, but the water just wasn't moving through it quick enough.
Fire: Because the tools for fire starting are so tiny and light, the list is longer but it doesn't take up much space or weight total. I carry a tiny metal tin in my pocket. The tin contains the fire tools and it can be used to burn things in low-oxygen (ie: making charcloth). If I haven't replaced items, I am sometimes missing some of what follows, but the general contents are intended to be: ferro rod, weatherproof matches (matches are optional if you have all of the other things listed), charcloth, tiny pieces of charcoal, a small piece (1 inch by half-inch, short enough to fit) fat-wood, a small piece cut off a magnesium block, a knife blade removed from a small pocket knife, a short length of a metal-cutting hack-saw blade which was cut from a hack-saw using another hack-saw (this gets a great shower of sparks from the ferro rod), often a little folded up piece of aluminum foil, sometimes a tiny tinder nest if I have one that small prepared, and a little scrap of paper with all these contents written on it to help me remember what I'm missing when I restock it (which can also be burned if necessary). All of this fits inside the tiny tin which is less than a half-inch thick, is about an inch and a half long one way, and about 2 or 3 inches the other way, and it weighs much less than 1 pound. Not sure exactly - probably 2 or 3 ounces. It just goes in my pocket, so it is always easily available.
The knife, water container, and fire tools I would say are the 3 most important tools.
Some kind of shelter is very important to bring. I said that the previous 3 things are the most important items because you can construct your own shelter from what's available, and that is fun to do, however your mobility is severely restricted if you do not bring anything for shelter, and your trip is less safe.
A tarp is an easy thing to bring. A small one need not take up much space or weight. A tarp is highly recommended even for minimalist camping. If you had not specified a 2-3 day trip, I would list this as "must have," but not taking one even on a 2-day trip is still silly. Obviously, you could substitute other things for a tarp, such as a bivvy bag for example. A tarp can also help protect you from sun exposure and heat in areas where that is a concern. Having a tarp for shelter makes you much more mobile than using only natural elements for shelter.
(Optional, sort of) A sleeping bag is a must-have for me, as I am in cold weather territory: when winter camping here you need to bring something rated for sub-freezing temperatures. A zero-degree sleeping bag is the biggest, heaviest thing among my gear that I would consider a necessity. It is annoying but life-sustaining. If camping in warm weather, this is optional, cold weather not optional.
(Optional) If you get caught out in the rain when you do not have a shelter up, a tarp can be sufficient to keep you dry, but it is big and clumsy to just wrap around yourself. For this, I would suggest a rain coat. That might sound odd for minimalist extreme-camping, but 1) you can easily get dangerously cold from the rain, and 2) you can get tiny emergency raincoats that take up practically no space and weight. I have one I bought from a sporting store which only cost a few dollars, and when it's still in its package it is only a few inches by a few inches, and less than an inch thick. No reason not to toss that into your pack; if you don't need it you'll forget you even have it in there, and if you do need it you'll be so happy to have it. I still have not opened mine, as the few times I needed it I was dumb enough not to have it, and I was miserable instead.
Shelter is more important than people realize. Without food you won't die for weeks, without water you won't die for days, but without proper shelter it is possible to die in hours if the conditions are terrible or even just from rain if it's not otherwise warm out. If the weather changes from bright sun-shiny to down-pouring rain rapidly (I've seen it happen very fast), and you're not ready for it, you cannot get a fire or shelter made fast enough, so you need to have something ready to immediately protect yourself. The alternative is to sacrifice your immobility and never leave a permanent (or at least semi-permanent) shelter.
Food: For the 2-3 days you mentioned, you can bring a little bit of food if you want, but you can just as easily live off the land if you are sure of your plant identification. You are not going to starve by eating nothing but dandelions for 2 or 3 days. The better you can identify (avoiding poison/toxic plants is more important than knowing good ones), the more food you can forage out.
For longer than a few days, yeah, don't forget to pack some food. If I was going ultra-light for a week or two, I would probably bring a granola bar per day and a bag of trail-mix, still supplemented with the plants as before. And maybe a box of pasta to eat halfway through. My diet is unconventional, but the general idea remains.
Another note on fire: Even if you get good at making friction fires so that you technically don't need fire starting tools so much, tools are still good to have anyway since starting friction fires all the time will wear you out fast and require you to eat more.
(Optional) Cord: can't forget the cordage! Although you can forage for plants to use for this, paracord is so useful and doesn't take up much room that there's no reason not to bring at least some, even if you don't plan on using it.
For minimalist extreme-camping: a knife, something for water that is cookable, something for fire, a tarp. This does not include special items that depend on the location or duration; other than such specialty things, this list is it.
That's only a few pounds so far, and if you carry the knife in your pocket or on a belt and the fire tools likewise attached to you somehow, that leaves just the tarp and water container to pack, so a bag is not necessary if you don't bring any of the items below - just attach the canteen with a biner or tie it to you, and tie the tarp up and wear the tarp itself as if it were a backpack.
If longer than a few days, then some food too. And depending on the climate, a sleeping bag.
Optional (but highly recommended): Water filter, rain coat, paracord.
Please note: none of this has mentioned first-aid, compass/map, or any other tools not directly related to your day-to-day needs that you are likely to encounter. That said, you should bring such things, and the above "optional" items too, even if you do not plan to use them. It doesn't hurt to bring along a few lightweight items. No matter how "bad-ass" you think you are (even if you really are one), it doesn't hurt to have a compass, map, minimal first-aid, etc. tucked into the bottom of a back-pack and to forget about them being there. It's the "not that you would, but that you could" mentality.
I will admit, though, that I usually do not bring compass and map. I have gotten lost as a result, but I just wandered around until I found a feature I could follow. However, whenever I have gotten lost it has not been in a huge park or preserve that you could get lost in for weeks, nor was it in a dangerous area; in such conditions I am much more likely to bring a map.