http://www.geocaching.com/ looks like fun.

"A GPS device and a hunger for adventure are all you need for high tech treasure hunting. Here you can find the latest caches in your area, how to hide your own cache, and information on how to get started in this fun and exciting sport."

Tap in your postcode an find all the hidden caches in your area.
Now how can I justify a GPS?, does anyone know of any DIY tools that have built in GPS. :-)
There is a cache in the park 2 minutes walk from my house, which is surprising.