Simple as it sounds in the title: potions have expiration dates. This let's players rest whenever they want without giving them many plot/story penalties ("you're too late hero, we've killed everyone in town") but instead apply direct combat penalties. As an added bonus, I can only guess that players would be much less likely to hoard potions, something they can often ease into. If somebody wanted to get very creative, certain expired potions could become acids, poisons, alchemic ingredients, bombs, or even entirely different potions. Thoughts?