Insead of having sandwiches or some depleteable resource, what if we have our Kerbals go on strike (stop providing SAS or specialist capabilities) after, say a week of mission time? To keep your Kerbals happy, they want a shower, magazine and music module.
This way we don't need to add a depleteable resource to the game and life support is addressed. Different showe, magazine and music modules keep different numbers of Kerbals happy. If they have the capability to shower, the Kerbals stay happy indefinitely. I like the idea of filthy Kerbals stinking up their capsules, anyway.
There could even be a "Jeb refuses to work until he's had a shower." message. Rather than sandwiches and oxygen, we could add a simpler, funny Kerbal morale mechanic.