A huge inflatable ball

Zorb seems to be a cool thing:

You can roll in it, run in it, ride in it, sit in it, slip in it, slide in it, lie in it, lounge in it, leap in it, live in it, swim in it, sin in it ... Jump in it, jam in it, move in it, groove in it, laugh in it, love in it. You can rotate, meditate, agitate, mutate, cogitate and invigorate in it. (...) How does it work? That's a person in the middle of this big ball. They've got about 700 mm of air between them and the ground - that's about 2 feet for you metrically challenged folks - and this air cushion keeps them safe as they hurtle down the side of a hill at speeds of up to 50 kilometres per hour. The Zorbonaut is strapped into a harness and rolls down the hill. This gives you maximum G force as you tumble head over heels.