At the Kellogg's Website I found a page that lists all their promotions! You can find out what toys are in which cereal boxes, enter a sweepstakes, or learn how to order one of their many mail-in offers. The mail in offers are great- all require proofs of purchase (UPC) from Kellogg's products (but you could get some of these from friends) and though some are free, some require a "postage and handling" fee. Still, it is neat to see you can order a Shrek bowl or straw, a Little Mermaid fleece blanket, Cholesterol test kit, T-Shirt, or even an inflatable Boogie Board! There are lots of offers here, and I think I'm going to start collecting cereal box UPC's!
Kellogg's Promotion Page