Well, first off, let's understand that just about any system that replaces the 8-hour rest cycle isn't going to offer the same level of simplicity. Having said that...
I mentioned an "action point" system previously. I was working on such a system when I was trying to create ad20 game based on the superhero genre (back before there were any worth a damn). Obviously, superheroes don't stop to break camp in the middle of an adventure, and the typical team of heroes doesn't have party healer, so instead characters have a reserve of points that can spent to give them a "second wind" of sorts, allowing hit points and certain special abilities to be replenished.