I've been browsing through some of the Android apps out there. In particular there's an interesting open source zombie game and andnav here on this site. It all got me thinking. And while I don't have the time, experience, or ability to complete the following project, I need to say it out loud.
Android Capture the Flag
Imagine looking at your phone to see your city divided into two sides with a neutral zone in the middle. Your location is marked on the map as well as small icons near you denoting your team mates. As you physically travel (walk, bike, car - careful!) to the other side of the city, you notice little red icons denoting players from the opposing team. As they get within your "visual range" they appear on your screen and you appear on theirs. Get within a certain distance (500 feet?) of an opponent and they tag you - you're out of the game. Get within tagging distance of your opponents' flag to capture it, and carry it back to your side for the win!
Capture the Flag would be particularly fun, but all sorts of schoolyard games could be adapted to use the GPS function on a large scale.
Impractical - yes - but fun to think about.