Another GPS-based game by the same creator as Geodashing that you would probably really enjoy is ShutterSpot: http://shutterspot.gpsgames.org - figure out the exact location where the photographer was standing when they took the set of pictures (or take a set of pictures & let other folks figure out where you were standing). There are a couple in Boston for you to find (they're both pretty easy), and I'm sure with your eye for interesting photos, you could create some cool ones. If you want some pointers on what makes a good ShutterSpot, let me know.