I third the cape and mask. The photobooth sounds fun and will make another awesome gift for the thank you cards.
As for plates, etc. I bought some hard plastic plates at the end of Summer at Target. They are bright color striped. They are now my birthday plates. I got 8 of them and they are used for parties only. You might be able to find some fun colored ones and then just do water bottles or plastic reusable cups. Buy for one party and use them for lots to come.
