* How about going was the painting "Whistler's Mother"? You would wear a black dress and the white scarf. See:
http://en.wikipedia.org/wiki/Whistler's_Mother * Wrap a foot in an ace bandage. Get some mittens or ski hat. Instant injured skier costume!
* Drummer Costume. Attach containers (e.g ice-cream containers, empty paint-cans) to a wheelchair with rope, duct-tape or similar material. Use a hoola-hop with card-board for the bass drum (over the front of the chair). Use paint-brushes or large pencils for drum-sticks. Dress in appropriate attair for the music to be played (e.g. rock, 50s etc)
* Fred Flintstone and car costume. Obviously dress as Fred with a pre-purchased costume. Create the sides and top of the car from cardboard or light wood. Ditto for the dash. Cover the side-wheels in grey plastic. Voila, Fred Flintstone driving his car.
* King or Queen and throne costume. Wrap back and rests with gold or silver paper. Attach garland, plastic jewels and the like to the cart. Dress up regally in velvet or velour. Buy a crown and scepter and even the costume at a costume store.
* Pumpkin Costume. Get a big orange leaf bag, cut a hole in the large end and put it over a wheelchair. Instant pumpkin!
* Robot or evil machine type of a costume.
* Dress up as an astronaut, and make the wheel chair up to look like the moon rover.