Some gems from my uni's freshman scavenger hunts in san francisco:
-a photo of a team member mooning the Blue And Gold Ferry (because they have the colors of our rival uni!)--it was funny that we were mooning an empty ferry, until a ferry packed to the gills with tourists scooted out from behind the empty one and we traumatized all those innocent folk who just wanted to take a boat trip to sausalito!
-a
durian fruit--10 pounds of stanky spiky death in fruit form, most americans don't recognize it, it's incredibly hard to locate outside of SE Asia, and you have to barter for it because the little old ladies selling it won't speak english.
-A mural painted by the same artist who painted our own dorm's mural, or a mural with a specific thing painted on it
I think the really fancy adult scavenger hunts that are run at some colleges are really elaborate 'quest'-style affairs, a la The Amazing Race. People have to collect a clue before they can move on to the next clue. For some, accessing the net to search on the go is a must. I don't have a link but surely you could find some for inspiration.