Make-your-own costumes...what comes to mind? Sewing, elaborate, taking lots of time? It doesn't always have to be that way, right?!
Many costumes can start with a sweatshirt or a box. My favorite costume was in 4th grade; I was a pair of dice with my friend Kerry. She and I painted the boxes, pasted on paper dots, and wore tights and a turtleneck.
A Baltimore Sun article with lots of links
About.com has costumes and family crafts too
HGTV has 3 minute video clips that include no-sew costumes and toddler costumes