There are many tutes on the pillowcase mattress. Here's an easy one.
http://bitofblueskyy.blogspot.com/20...-tutorial.html
Other ideas- Zippered bags, crayon totes, sweatshirt jacket, anything personalized, origami bags for toys (google for many directions). Their own special dinnerware and placemats? Laundry bags? Anything that encourages cleanup after playtime. Have fun and show us what you come up with.