Have you ever been? We started a few years and recently got serious with it and bought a premium membership. What is it? Its like a treasure hunt, you pull up the coordinates on a GPS device/cell phone and it points you in the right direction to where hidden boxes are. Inside those boxes are little prizes that you can take and replace one back yourself. We have done it a few times this summer and love it. You log your name away on a paper along with the date when you find the box, the boxes are always well hidden from other people who are no geocaching.
Here is the main website if interested on more information http://www.geocaching.com/
Caleb looking for cache
This is on the back of my car window