Playing games while moving around in your town with an iphone-like
device (and having your location matter in-game.) You could port any
kind of game to this concept: counterstrike, racing, even monopoly. Or
pac-man, chasing your friends around the block. Or Sokoban, pushing
giant boulders through Fifth Avenue.

(reply) I had this idea six years ago. I'm appalled that it still
hasn't happened. Of course, very few devices have GPS chips, and
operators are wary of sharing the location of their customers with
random companies. But still!