I can only think of the epic scavenger hunts that could be had with the Reverse Geocache. The Reverse Geocache was built by RickP, an Arduino forum member who designed the Reverse Geocache to only open up at a specific GPS location.
The Reverse Geocache will look for a GPS signal for three minutes. It will then display how far players are from the target GPS coordinates in kilometers. I don't think that I'd have the patience to play a game with this Geocache, but it certainly is a cool creation. GPS gamers, this is one you don't want to miss.