A year ago I hid a disposable camera in screw-top container in a tree stump in a local park, and left instructions on the internet for people to find the camera, take a picture of themselves, and put the camera back.
Yesterday I retrieved the camera and had the pictures developed. You can see the result here.
What an enjoyable experiment! A montage of people (and dogs) - most of whom I have never met, all having fun.
If you've never tried it, I'd heartily recommend geocaching (http://www.geocaching.com) - probably the most fun you can have with a GPS receiver. There are thousands of hidden "caches" like this all over the world - probably just down the road from where you live.
I've put another disposable camera at the same location, so if you feel like getting outdoors and having some fun, try finding my geocache called "Only the Lawnleis". You'll need to discover a little bit about the local history of the area before you can track down the final location. It's all on the site here.