Depends on the activity. Depends on the time of year.
Summer: Water. Don't bother with anything else. Salted nuts are part of my lunch and there is a salt shaker with supper. Since in my hiking country stream crossings are rarely more than an hour apart, water is convenient. A 500 ml bottle is fine.
Winter Snowshoeing. Water is much less available. I used to organize cross country orienteering contests, so would often spend all day putzing about setting controls for meets that were far enough in the future that my tracks would vanish.
Anyway, I would take 2 liter pop bottle full of hot coffee with lots of milk and sugar.
Dogsledding. We usually traveled 2 or 3 per sled, with half the people at any given time up front breaking trail. For these we carried 6 quart thermoses that we filled at breakfast with juice crystals (faux juice) made at double strength with boiling water.
In use, you would fill the cap or a cup with snow, add hot juice and turn it into double the volume of warm juice, or into a slushy if you preferred.
If you are into performance endurance racing, the standard faux juice is too sweet, running about 10% sugar. You want a solution around 5-7%. This is absorbed fast by your gut. Depending on the weather you can tune this. Hot weather (sweating due to environment) you want less sugar. Cooler weather (almost all sweat due to exertion) you can use more sugar. Fill your camel hump with the right level of glucose (not plain sugar) and you can postpone depleting your liver's glycogen and delay hitting the wall.
I have found running trips with teens that, without flavouring, many kids will fail to drink enough, become chronically mildly dehydrated, with the attendant mild headaches, grumpiness, and lack of energy. To keep them hydrated: Served hot chocolate with breakfast, dilute faux juice at lunch, and strong sweet tea at supper, as well serving soup as part of supper. In most cases all of these were served at about twice the dilution you would find in 'civilization'.
Yes, they drink when really thirsty, and will gradually start drinking enough, but that takes a week. Meanwhile at every stop, you have to remind them to drink.