Take one unwanted CD (or DVD) and the cap off a sports bottle, the kind that you pull and push the top to let the water out.  Attach the sports cap over the hold in the middle of the CD with blu-tak, or similar (to make a leak proof connection), and close the cap.  Blow up a balloon (we in the trade would say ‘inflate’ the balloon), and ease over the top of the sports cap.  Now place the CD on a smooth surface, open the cap (without removing the balloon), give the thing a small nudge and watch what happens.

Bingo!  One very small hovercraft!  Took it into playschool, where Joanna said they’d made them at Beavers, but she’d forgotten how to make them,

(I got this out of my Physics World magazine)