One of the fun things we first did when GPS hit our WM phones was to Geocache. Geocaching is a game in which people leave small items semi hidden with gps coordinates for you to find. An excellent starting place is Geocaching.com.
I signed up for an account and found at least 50 caches with in a bike ride of my home. I spent much of last summer locating and tagging the caches I located. It’s a nice way to break up a bike ride or walk.
The TUAW has an article on how you can use your 3G iPhone to join the WM mobile users in Geocaching.
If you have an iPhone it’s just as easy. Open Google Maps and type the lat and long into the search box [ N 30.234 W 90.234 ] and it’ll bring up your cache location.