We went to the LA Zoo yesterday and had a picnic lunch near the kids' playground. It was in the grass under a shady tree. We brought along this 80" x 60" picnic blanket ($12.74 on Amazon). It folds up into a small package and comes with a handle for easy carrying. The underside is made of a tough material to resist scratching, and the top is softer and easy to wipe down. It has a thin layer of foam to provide a bit of a cushion. We've used it a bunch of times and it holds up well.
