I just picked up one of these waterproof blankets at Target this evening while doing some last minute Christmas shopping. Got home and unpacked it. Looks like it will work for either a padded seat, pillow, blanket or throw.
One side is slippery smooth and appears water resistant (don't know about the waterproof thing). The other side is soft. Size is about the same thing as an unfolded rectangular sleeping bag. Might just use it for spring/summer camping instead of a full sleeping bag. $20 is a good deal for anything made by Kelty.