A tyre swing.
Spent a lot of time swinging on this, I did.
You can't see it, but the embankment falls away to bugger all, leaving you swinging out over a drop of about 20 feet.
I remember being simultaneously thrilled and concerned about this, being the sensible little earth sign that I was, but must have decided the risk was worth it, allowing me, as it did, the opportunity to fly off to the most outrageous locations. In my head, anyway.
Provided, of course, that I remembered to aim away from the tree.