The other day I found an article online about this game called Ingress. The basic premise is that there is this weird energy seeping into the world though "portals." Portals are located near statues, historical locations, and places of cultural interest like museums or churches. There are two teams who are fighting over each of these portals to capture them and harness the energy.
This game is interesting because you have to physically move to these locations to be able to do anything. You have to do additional walking around to gain energy to be able to complete actions at these locations.
I've been looking for a GPS based game ever since I got my iPhone. I live near the Duluth Lakewalk and there are plenty of "portals" located along it. So all I have to do is go down there and I can play until my phone goes dead. This is only about half of the area I walk to. I can walk further down the lakewalk and I'll hit Canal Park and there's just as many portals there. I accidentally walked down there the other day and I found I can get there in an hour if I'm walking at a regular pace.