Letterboxing is like geocashing, except with stamps.
This summer my friends and I carved out stamps. After finding directions for letterboxes by Enger Tower, we traveled up there and started looking around. We found the first box, and, within it, there was a stamp and a notebook. We stamped our stamps in the found notebook, and stamped the found stamp in our notebooks.
This is the main website that contains the locations of many of the boxes.