I save cereal boxes (and other similar shaped boxes) for mailing photos or other papers I don't want crushed during transit. I cut out two pieces of the box, put the paper or photo between them and then tape around the edges. Then I put it in an envelope and mail. Eventually, it will still end up in the garbage, but I feel like at least I reused it once.
I also use cereal boxes to make "books" with my DS. He loves to make books using two pieces of the paperboard as covers. You can either staple them together with some paper, or punch holes through them and tie everything together with some yarn. Let him draw on the covers and write the title and your child's name as the author. I'll be saving all of these forever!
Can you reuse the beer carriers for silverware at picnics or buffets? I don't know if they'd hold metal silverware, but I reuse our plastic silverware when we host family get togethers and a carrier might hold them.
How about using the carrier to hold colored pencils, markers, paintbrushes?