Letterboxing is an activity that's very similar to geocaching but much older.
From Wikipedia, the free encyclopedia:
Letterboxing is an outdoor hobby that combines elements of orienteering, art and puzzle solving. Letterboxers hide small, weatherproof boxes in publicly-accessible places (like parks) and distribute clues to finding the box in printed catalogs, on one of several web sites, or by word of mouth. Individual letterboxes usually contain a logbook and a rubber stamp. Finders make an imprint of the letterbox's stamp on their personal logbook, and leave an impression of their personal stamp on the letterbox's logbook—as proof of having found the box. Many letterboxers keep careful track of their "find count".
A letterbox hybrid type geocache has elements of both a geocache and a letterbox. It's found using a GPS and it contains a rubber stamp.This cache is a small, sandwich sized, Lock&Lock. It contains a simple hand carved stamp and stamp pad. Please leave these in the cache. They aren't trade items.