These clue ideas work very well for adults doing treasure hunts:
Drink a gallon of water...or something stronger! - this is where treasure hunts meet drinking games, with shots or some other alcohol sitting between the hunters and their next clue. And for the non-drinkers, you could swap in something that's hard to stomach, like Clamato juice (that's clams and tomato juice).
Scavenger hunt - The treasure hunters are given a list of items that they must retrieve (e.g. a lit match, Ace of Diamonds...). If they retrieve a certain amount of them within the given time limit they get given the next clue.
Miming letters - Messages are mimed one letter at a time from one person to another through different rooms.