This is something I've never tried, but I think it's something my little nerd self would enjoy, especially as a youngster. Here are some links and tips to get you started:
What is geo-caching? It's an outdoor treasure hunt using GPS-enabled devices.
- Official Global GPS Cache Hunt Site
- GeoCaching 101
- How To Go Geocaching with Your Children
- Geocaching with Kids
- Geocaching-Themed Birthday Party
(Image: Flickr user climbNH2003 licensed for use under Creative Commons)