A tent:
A tent the player can take along, giving the ability to rest, or the tent can be left at home, but then no resting.

There's some stimulation here for players to play effectively - to rest less.
In the current game the tent is already graphically represented when you're resting, so this idea would fit in easily.
No XP from monsters spawned by resting:
This might spoil it a bit for some, as I've seen threads where players are going on about their ideal camping spots for XP gain, but it's a bit of an exploit, isn't it?
I'm currently playing a game called Divine Divinity, not too unlike Eschalon, and this game's got a unit called Skeleton Conjurer in it. Get near this Skeleton Conjurer and it spawns skeletons. Of course it's a baddie, this unit, and at first I thought I was clever by letting them live for a while, so that they provided me with more skeletons to kill, until I figured out that these conjured skeletons didn't give me a single XP point!

You could translate that to what's happening in Eschalon with monsters spawned by resting. I personally find that would be better - no XP from them, keeping the gameplay straight.