Geocaching. It's a funny looking name but it has millions of hidden caches(hmm, that might be redundant, or worse, the same thing said more than once for no particular reason).
There is a game called geocaching. It is a chance to find hidden items by means of geo-coordinates and clues to a specific area to locate a small box or even a 35mm camera film container(pretty small).
This is fun for all ages and abilities, with easy to locate caches, to mystery type caches with multiple clues, to very difficult climbs for the cache. Check it out at www.geocaching.com
LiveCampWORX for the adventurous and the curious.