Simple idea. Someone hides a box with a logbook and some goodies in it. Then they publish the GPS co-ordinates of it. Everyone else then tries to find the box. Finding the box could be simple or tricky; getting to the box may be quite hard. If they do manage it, they sign the log and exchange goodies leaving the box where they found it.
This geocaching has grown to a bona fida sport with active communities searching for over 171,000 caches hidden in 215 countries! Sound like a brilliant excuse to buy a GPS unit, meet new people and get some exercise (or take the jeep out for a spin). I'm tempted. There's even a UK Geocaching website.