Decorate a lunch box. These are available at Oriental Trading for $5 which is actually the cheapest I've seen them, but you'd have to look at shipping. They are available at craft stores everywhere as well. They can be decoupaged, painted.. whatever you can dream up. And, it would be fun to send home some fun school supplies as party favors. Everyone loves new Crayons and Target has them for around 25 cents right now. .. things like that.
If this seems a little advanced for your party goers.. how about a lunch bag?
Make a paper bag puppet.. You can be as simple as handing kids a paper bag and some crayons or as elaborate as the example above. I thought these lions were cute, but you have to see the elephant over at ScrapbooksEtc.com