Letterboxing is an intriguing pastime combining artistic ability with "treasure-hunts" in parks, forests and cities around the world. Participants seek out hidden letterboxes by cracking codes and following clues. The prize: an image from a miniature piece of art known as a rubber stamp—usually a unique, hand-carved creation.Check out Atlas Quest or Letterboxing North America for more information and clues!
Letterboxers stamp their discoveries in a personal journal, then use their own rubber stamp, called a signature stamp, by stamping it into the logbook found with the letterbox, perhaps writing a note about the weather or their adventures in finding the letterbox.
I'm just going to cut and paste from my favorite Letterboxing website, Atlasquest: