I had capes made of satin with Velcro closures. Each kid got to choose a brightly-colored cape and then decorate it with cut-out shapes made from sticky-back felt. This served as a fun activity and a party favor too.
I purchased this image from http://www.etsy.com/shop/kottageon5th?ref=seller_info and used it for invites and thank you notes that were "Super" cute! This is a card describing the refreshments, in this case Super Hero Energy Fuel.