1. Make holiday cards/love notes for family and friends. (At Valentine's Day, we used a Cheerios box.)
2. Make a puppet theater
3. Cereal boxes (and other food boxes - cracker boxes, etc.) are great for getting big, thick letters that pre-schoolers can use to create words. They can lay them out in muffin tins or on the table and keep switching out and creating different words. Then, there's no glue required. (Yes, yes -- I know there is the association with ransom letters, but your little one won't realize that).
4. Make a placemat. There's two types that I've seen: weaving cut box pieces together or cutting the box down to the appropriate size and covering with contact paper (photos or other designs/images can go between the cereal box and contact paper).
5. Make a puzzle.
6. Make a village. You can use decoupage, foam and go with a fancier version, like these examples. For children that love paint and messy play, this example looks great.
photo from Dinner Loves a Story
7. Make a postcard - I'm sure grandparents would love to get a postcard made by your child using a recycled cereal box.
8. Make a notepad.
9. Make bags (purses, tote bags or gifts bags) - with fabric or with paper.
10. Make backseat boxes.