it would be fun if there were hidey-holes here and there, in out-of-the-way locations. they could be in tree trunks in locations that you had to do some tricky jumps to get to, stuff like that. you wouldn't be able to see that they were there until you were close enough to get them to light up. stuff could be left there, either generated by the game, or players could leave notes and goodies for others to find, a la boo radley. maybe some could work like geocaching, where you'd have to leave something behind and add a line to the associated note.