Do you have simple rules for exposure or hunger and thirst?
I'd like something simple enough that players can remember (important when they are making choices and don't require constant look ups) and that take as little bookkeeping as possible. Here are some draft ideas:
A character needs a ration of food and a wineskin of water each day.
I think the week as a unit for rations is not fine enough-- how often do you get through a week in a single session? That means you are going to have to track partial rations. For water, a wineskin is quite a bit, but if you consider someone traipsing about in sweat-inducing armor, taking part in combat, and hiking for miles, I think it's plausible enough to use for simplicity's sake.
I need to look back over Talysman's simple hunting rules because I'll probably use them. One idea I had this morning was:
Each hit die a game animal has can supply one ration of food.
Again, might seem like a lot of meat is getting wasted but it's simple and the warriors are hungry. As for exposure, what if we keep temperature to 3 levels above or below the norm. And have a few things that ameliorate them. Cold is easier. You could have heavy clothes, shelter, or a fire. As it gets colder you'll need combinations of the three. So, it would look something like this:
normal = nothing needed
cold = one aid
freezing = two aids
bitter cold = three aids
So, for stage 2 coldness you could huddle in your heavy furs and shelter, or dress normally in your shelter with a warm fire, or brave the winds with a fire and heavy furs.
What's the penalty for not having the appropriate gear? Maybe -1 hit point per hour? Or to make even high level characters fear the wilderness, - 1 hit die per hour exposed. For heat I guess shade, a body of water, and maybe shelter too (underground or something) could work. It might be okay to make the heat less survivable, seems to be true in the real world.
I would want to figure out food and water requirements for beasts of burden. And maybe set a threshold of cold/hot they can stand. Outside of it they take the same damage as players. That way it would make llamas and camels equally interesting in different parts of the game world.
What else? Those would go a long way to adding tension to exploring harsh terrain.
The only other thing I can think of is sleep. But that is much more complicated. It often happens off stage, it is by default boring (nobody is making any choices), and fiddly to determine if sleep was good or bad: was there too much noise, is the ground rocky, did you feel safe enough to rest soundly. Maybe a better way to handle it is you need to camp every so often or you get some penalties. But that's pretty much the default for D&D now, no? No camping, no healing, no spell memorization. So maybe we don't need to worry about sleep at all. The need to camp is built into the game. And a party harried in the wilderness, trying to find shelter from the cold will probably not camp if they can't find shelter etc.
If you have ideas or know of good simple rules that add to the verisimilitude of characters trying to survive in the wild share them below.