Last year, my girls went as super heros: Batgirl and Wonder Woman.
Wonder Woman's costume was made from red and white felt and Velcro. I found blue boxer shorts in the boys section of Target and adorned them with white felt stars. Gold paper was reinforced with clear mailing tape and used for the cut out stars on the top, crown and belt. Simple gold rope was looped and secured with more Velcro on the inside of the shorts.
Batgirl's costume was even simpler. A black turtleneck leotard and black leggings were transformed into Superhero status with bright yellow and black felt. The "boots," which are worn over my daughter's regular shoes, were the most time consuming part of the whole costume.