Bunker

The other day, I came across an awesome kid's idea for the garden: a secret bunker. You basically dig a huge hole, fill it with a concrete box and cover over it with earth and plants and stuff. Not exactly original, but with a bit of imagination you can make a space that kids will love and - unlike a treehouse - they can't fall several metres and break their leg (although when I get round to it I am going to make a treehouse for JKY as well). 

All I need now is big enough garden...