- Players traveling in less than ideal conditions should risk daily damage to Hit Points.
- The damage must be real Hit Points and heal in the standard manner - no extra bookkeeping.
- The damage should scale, so that high level characters do not outlast low level ones to a ridiculous degree.
- Easy to remember.
So: Damage can be taken from Bad Going as well as Bad Weather. Each day, check damage in the mid-afternoon based on the average events that day. Each character rolls 1d6 per 10 hit points he or she currently possesses*. A roll less than or equal to the Hazard Number means you take damage equal to the amount of the roll. At less than 10 hp, you still roll but take half damage (round up).
* (Or for an Alexis-inspired variant, 1d6 each day, plus an additional d6 per 3 full days gone by without spending a day or night in a safe place.)
What's the Hazard Number? Take a base of 0...
Worst Going (swamp, dense forest, mountains): +3
Worst Weather (2 on a 2d6 weather roll; pouring rain, blizzard, baking heat): +3
Bad Going (hills, woods): +2
Bad Weather (4 or less on 2d6; raining, snowing, hot and humid): +1
Average Going (plains, countryside): +1; Average Weather: +0
Following a trail, road, or a guide who has successfully used a terrain-lore skill, reduces the badness of going by one category. Appropriate gear for protection reduces the badness of weather by one category.
Being at 0 hp or less lays you out from exhaustion. Hit points are recovered normally from rest and healing (I allow 1 hp/level/day for rest in a secure place), but camping without shelter disallows recovery and puts the party at risk for more bad weather damage.
I'm not sure the "cleric problem" is that much of a problem, if healing abilities are seen as being as much about restoring morale and cheer as curing physical wounds. Taking a divinely inspired priest or druid along on an arduous journey should be seen as a good idea. If monsters and more trap-like physical hazards are popping up along the way as well, the risk of taking damage from an additional source every single day will just add to the strain on resources.