http://www.geocaching.com/
Basically you download coordinates into a GPS receiver and go there trying to find "caches" of stuff left by other people. Some are just logbooks that people sign (take various forms), others are bona-fide caches of stuff. Usually trinkets and stuff but occasionally something semi-valuable. Typical courtesy is to "take something, leave something".
A lot are very difficult to find and hidden in plain sight all over the place. I was shocked to learn how many are out there!