I make my kids' costumes, but figure since I'm doing all the work, they'd better be useful, so the rule is that they double as pajamas...usually from sweatsuits.
One year I used a pair of old sweats and sewed netting and a few extra "arms" under the arms and turned it into a spider.
One year I sewed a black cape over the back of a sweatshirt and he was a bat.
One time I glued silk "leaves" to a red shirt and my son wore brown pants and was a tree.
Then my oldest son convinced his little brother to be a piece of paper. People thought he was a ghost until he held out his arms and you could see the lines and "holes."
My favorite was gluing a bunch of straws (one...at...a...time, sigh...) to a sweatshirt to make my daughter a porcupine costume.
This year my son says he will be a ranger. We already have a nice fleece hooded cape, so he'll have a belt around it and stuff toy arrows and such into the belt and call it done. I'm too tired to care this year! And I'm all for simplicity.