This was a collaborative project between me and a good friend who likes fiddling with LEDs. It was his idea, based on how once the dew comes down lying on the grass at festivals is no fun any more. He suggested a waterproof bottomed picnic blanket with a removable-for-washing top covered in interestingly textured fabric in a trippy pattern.
The LEDs were just a bonus, although Jez might tell you different. They really do make it easy to find in the dark though.