Hidden Park

“… it’s a [mobile, GPS-based] computer game that gets the kids off the couch and playing outdoors. The application uses all the features of the iPhone to lead children into a fantasy world of trolls, fairies and tree genies – right in their local park! […] you can set up a version of the game in your local park, then share it with friends in your neighbourhood”

Although you’ll have to trek to London (Kensington Gardens) to try it in the UK, at present.