This is arguably the coolest out-door activity ever. And it ranges in difficulty level, so pretty near everyone can geocache. The game goes like this. Someone puts something in some sort of box, and hides it anywhere, preferably in the woods. This person can then either post the exact coordinates of where they've hidden the box, or create some sort of treasure map. The coordinates/maps/clues are posted online, and geocachers go hunting. If/when they find the cache, they open the box and replace the items (or one of the items) with an item of their own. A good cache would be a CD, some sort of craft, or other little stuff. Food probably isn't the best idea. I've heard of a disposable camera being put in, and when you find the cache, you take a photo of yourself. When the camera is full, the pictures are developed and posted online.
I thought since most of you are England based, that this could be fun.