Basically a treasure hunt of sorts... no grand prizes and not many treasures. You need a GPS an an account at GeoCaching.com to find them. Coordinates are loaded into your GPS and you go find the cache, log that you found it, and do it again. If you are the adventurous sort, you will find yourself in places that you never would have found otherwise.
There are other types of things you can do, like what we did on August 5, 2006. I gave a bead to my hubby and told him I wanted it to go to Murano, Italy and then back home to me here in California. It's called a Travel Bug. So, he tagged up my bead and dropped it off to start it on it's way. It took my bead 4 years and 8 months to get to Murano. Italy. It traveled 19667.9 miles before it landed in my house this afternoon. A kind man from Mesa, AZ dropped it into a mailbox and sent it home to me before it could be picked up and sent away even further (most likely) away from home. Lampwork Bead TB (Travel Bug) started it's journey in California before heading to Virginia, Georgia, Italy (several times), Austria, Germany, Switzerland, Monaco, France, California, Arizona, and finally back in California. I'm going to start a scrap book for it.