Geocaching does, indeed, seem v. nifty.

I like letterboxing as it doesn't require a person to have a GPS unit.

Have you heard about the Vertigo inspired letterbox/scavenger hunt scattered across San Francisco? I'd dig up the URL for you but a) I think it might be inactive and b) my virus-scanner is hogging machine resources but if I pause it again, I'm afraid my machine will choke and start all over again. More later. ;)