A year ago we had a superhero party. I used pieces of flannel, cut with pinking shears as capes. The kids just tucked the capes into the back neck of their shirts and voilia! Superhero capes. If the capes slipped a bit, I fastened it with "cape clips" (those metal hair barrettes that you flip into place)
Being as I also have TWO superhero affectionados in my house--may I recommend that you also make a few red capes? You'll never know how often they come in handy and it's nice to have a few extra for friends and siblings.
Baby Bunny Blanket
Feather and Fan lace scarf