This fun idea comes from a crafty friend of ours who made this as a gift for B. She took a plain T-shirt and ironed on a superhero badge, then she hand sewed a rectangle of sparkly, sequined fabric to the back neckline as a cape. Instant superhero!
If you can't find a superhero iron-on, you could use some fabric markers to get the same effect.
B hasn't decided what she wants to be for Halloween, but this is a contender. Do you know of any other easy homemade costumes?