Dusting mop or tree branch with aluminum foil fitted to the end.
Have an adult cut holes in the bottoms of four small disposable drinking cups. Use duct tape to fasten pairs of cups together, bottom to bottom. Optional: Draw buttons or controls on top of cups to control virtual zoom or infrared. Use a shoelace for a neck strap if desired.
Wrist Control Panel and Bullet Deflector:
Create arm bands out of old kids' socks by cutting off the feet. Draw superhero buttons on the arm bands to ignite imaginary rockets or lasers. For an inter-galactic look, try making the wrist guards out of paper covered in silver duct tape. Cut to desired size, add another strip of tape to secure on your arm (silver side out) and draw on the necessary controls.
Old Adult tube socks are great for adding some flair to a costume's legs. Cut off the sock feet and pull them up your child's legs as high as you can, to make super power speed leggings. You can emphasize the tights' speed power by drawing lightening strikes down the sides.
It was said that Mercury was the God of Speed, and that he had wings on the backs of his feet. Shape a few short wings out of tin foil and slip them in the back of normal tennis shoes to confer Mercury-like superhuman speed.