If something goes wrong, the information you provide your emergency contact will be the starting point for the information Search and Rescue groups use to look for you. Much of this information will remain the same for multiple trips and some will be trip-specific. The items I list won't be comprehensive, but rather a selection of items from an online trip-report form that was created for just this purpose. Thus, you can prepare in advance a packet with most of the information and just update the trip details each time you head out.
- Name, age, sex, height, weight, hair and eye color.
- Colour of outer clothing likely to be worn.
- Any medical conditions, medications, or allergies?
- A recent picture of yourself.
- Are you a skilled hiker, a novice kayaker, regularly climb 5.13 trad or stick only to trails?
- How much first aid knowledge do you have? What about navigation and survival knowledge?
Your emergency contact should know when you will be definitely overdue, and who to call for assistance.
A definite time helps avoid undue delay and unexpected/unwanted rescue when you thought you were well within the expected time.
Although it's always possible to call 112 (or other emergency number), a swifter response will usually be obtained if you can provide a number to call that's specific the area where you'll be.
Make sure your contact knows what to expect when calling the emergency service (e.g. in the UK, mountain and cave rescue is initiated by Police, but for maritime rescue, we need to ask for Coastguard instead).
Are you taking a car, bus, train, plane, or llama to the trailhead? For personal vehicles, the make, model, color, and license plate number are helpful. (If you take a bus, the driver or other passengers might remember you.)
A map with your expected route highlighted is most convenient. Alternate routes, expected campsites, and potential side trips should also be noted. You should have a rough idea when you'll start, when you'll return, and generally how quickly you cover ground.
What navigation or signalling equipment do you have? If a PLB, is it registered? If a cell phone or two-way satellite messenger (InReach, Spot, etc.), what type is it and what's the contact information? Is there a URL where it displays its location? Other communications equipment you might be carrying could be a torch or signal flares.
You don't need to list every item, but larger items are easier to spot and smaller items may be found by searchers. An easy way to inventory is to simply lay everything out on the floor and take a picture. While you're doing this, it's also good to note your shoe size, model, and include a picture of the soles.
Rescuers will also likely be interested in the things that will affect your ability/rate of progress - e.g. crampons, torch, spare paddle.