The Den

 
Geocaching is about the only thing I've every done with my old-style GPS receiver.

Enter your location at this website (zip code, city/state, coordinates...) and it will show you all of the "caches" you might be able to find nearby.

A cache is (usually) a small waterproof container, camoflaged and/or hidden somewhere interesting. You are given the coordinates of the cache, and perhaps a clue or two, and off you go on a treasure hunt. Often there are little toys and trinkets inside (if you take one, leave one) or other odd things like 'Travel Bugs'.  Its not always as easy as it sounds. Check out the website to learn more.