This was so easy, and the kids tell me they're doing it at home! Have each child bring you an empty cereal box from home. You can get fellow teachers to donate some of theirs for those children who don't bring one. Cut off the front, then cut into "puzzle" pieces. Store in a plastic baggie. Keep the bags in a box for them to sort and pick from. (Hint: use a paper cutter instead of scissors. Much easier!)
Extra hint: later on you can re-cut the pieces into smaller sizes to increase difficulty!