A hand-held device with either accelerometers or a GPS receiver (probably the latter), that can randomly generate mazes that it stores in memory. You walk around holding this device, and when you hit a wall it does one visual/sound, and when you find the exit it does another visual/sound. So you basically navigate this invisible maze until you make it out.
(it won't let you walk through walls -- if you try it'll just keep giving you the ugly noise until you get back to within eyeshot (within the maze) of the point at which you transgressed. or alternatively, it'll scrap the maze and you'll have to try a new one.)
Should be able to specify dimensions (in feet) of the maze and maze complexity (which determines thinness of the corridors).
i've changed my mind. don't just generate a random maze. take one from a database, or seed from a random number in a database (that implies you don't change the generation formula). also, perhaps have a way of networking maze entries among your friends, etc. what i'm aiming toward here is Sheldrake's morphic resonance.
this can most easily be issued as an iPhone app.
but you'd better hurry. i purchased my first iphone today.