"Letterboxing is an intriguing “treasure hunt” style outdoor activity. Letterboxers hide small, weatherproof boxes in publicly-accessible places (like parks) and post clues to finding the box online on one of several Web sites. However, clues to finding some of the most highly-sought boxes are passed around by word of mouth. There are about 20,000 letterboxes hidden in North America alone. Individual letterboxes usually contain a log book, an often hand-carved rubber stamp and may contain an ink pad. Finders make an imprint of the letterbox's stamp on their personal log book, and leave an imprint of their personal stamp on the letterbox's logbook ."
You don't need a GPS, just the ability to follow clues. Also, you create your own unique rubber stamp that allows you to sign the log books. I know my kids will love doing this.