Geocaching. This isn't anything new for some people, but this sounds like a ton of fun in a very geeky way. Who doesn't like a scavenger hunt? I think the best part about this is you might get to see some cool spots that you otherwise never would have seen.
How does it work? People create 'caches' in some out of the way place. The cache is filled with a bunch of worthless trinkets (keychain, screwdriver set, etc) and a logbook. Then they post the coordinates on the web (link above). The hunt is on. You can search the Geocaching site for caches near your home or where you are going on vacation. Enter the coordinates into your GPS and start hunting. Add a comment to the site and/or the log book in the cache.