Day and Night Cycle!!!!

Think of Lord of the Rings and the gates of Moria. The letters written on it were only visible in moon- or starlight. Many tales about undead also deal with certain times, e.g. that ghosts only appear after midnight.
I could also think of riddles in which the sun has to be in a special position to hit some spot on the wall (think of old Egyptian, Inca, and Celtic temples).

Plus, it would look sooooo awesome!
I doubt you will see a quest from us that requires you to stand around for 10 hours until the sun goes down...