TH - take a peek at www.geocaching.com

It's geek hide-n-seek. Someone hides a small container of trinkets somewhere, and gets the lat and long of that spot with their GPSr. Then they post a description of what they hid, clues to where they hid it, and the co-ordinates. Cachers get the info, go find the cache, and trade a trinket from the cache and leave one in it's place.

Some of them are really clever. Like "looking up" into a tree, where the cache is a big fake pinecone (in a maple). Some have math problems or riddles you have to solve to get the final clue. Some are just a plastic bag of cr@p tossed under a shrub.

The cachers who are really into it create a signature item, something they leave at every cache they visit. SKnot and I make zipper pulls with fimo beads as our sig item.