Apologies if this old news but I just learned about this a couple of weeks ago... www.geocaching.com

Basically, someone hides a "cache" somewhere (legally) and posts the coordinates on the website. You then enter the coordinates into your hand-held GPS and go find it.

I was surpised to find 3 caches hidden in the neighborhood park a couple of blocks from my house. It seems like good, clean, family fun.