We had a girl scout picnic over at our place yesterday, and we spread out the Mui Color foldable picnic blanket ($(removed) on Amazon). It folds out to folds out to 60" x 78", and folds up to a 12.5" x 10.6" rectangle complete with handle. The bottom is made of some kind of tough waterproof material. It's padded, and the top surface feels nice and has a pretty leaf pattern.
After the picnic, it was easy to fold up.