I recently came across this site Quick & Simple that had a fantastic DIY gallery of fun halloween costumes to make yourself on the cheap! 
Check it out: Quick & Simple


If you are really stuck and need a fast fix for an undecided child here are some suggestions:
- a mime (pictured above)
- a hippy 
- Bunch of Grapes (cover a tracksuit with purple or green balloons)
- a puppy or kitten --all you need is a tail and some ears!
- a pregnant woman -- just add a pillow!
- robot (cover 2 boxes with aluminum foil and wear them)
- chef
- princess
- rockstar
- spider (use stuffed pantyhose for the extra arms) 


....use your imagination the possibilities are endless