You probably already know about GeoCaching, which is a brilliant idea and a lot of fun.
Now, via Andrew Alden's marvelous Oakland Geology blog, I learned about a variation on GeoCaching, called EarthCaching.
EarthCaching is GeoCaching specifically focused on geology in the natural environment. Andrew Alden highlights several local earth cache sites, while the main EarthCache site has a short list of the Top Ten Earth Cache Sites In The World!
Say, I've been to a number of them: the Green Sands beach on the Big Island, the Thunder Eggs in Alabama, and the Merced River in Yosemite! I've flown over the Great Salt Lake, and I've been to Tuolomne Meadows. Haven't made it to New Zealand, Germany, or Portugal yet, though...