Me and my girlfriend used to do this a LOT. We'll start again this summer probably. All ya need is a GPS and get on that site, they have thousands of local locations, remote locations, every place you could think of and hundreds you wouldn't. It's a ton of fun!
They are usually found in ammo boxes. You take something, you leave something, and mark the time down in a notebook usually found within. I thought it sounded odd at first, but once you do a few it's kind of addicting!