When a costume is needed within a couple days, you can pull something together easily. They may require a purchase or two, but otherwise they involve using what you find at home or visiting a thrift store.
Some are fairly common costumes, but children do not care if it is clever, they just want to pretend to be something else. Easy ones like a ghost involves nothing more than cutting large enough holes for eyes and mouth if you wish in an old white sheet. You can cinch the waist a bit if necessary and make sure it is short enough to not trip over. You can pin some fabric at the wrist so they can wave their arms when spooking the neighbors. You can also just place fabric over the neck and wear scary white face paint and a white wig. Add a chain for another fun effect.
For a witch the only things needed are a black dress or skirt outfit and a witches hat and broom. Just make sure the outfit is adjusted to fit. You can make a hat from poster board. If you want to decorate the old broom in the garage wrap streamers on it. Make some scary hair from an old mop and use face paint. You can add striped stockings, a cat or a bat or a spider on their shoulder.
Maybe your little girl wants to be an angel. Thrift shopping should find you some pretty sheer or satiny outfits for your base costume. We used to make wings form poster board, but since the price has come down, your best results will be bought ones. The angel could wear a pretty long blonde wig or go with her own natural hair. Add a halo made from pipe cleaners and spray painted gold. Attach that to a head band for ease in wearing. Sprinkle clothing or anything else with a bit of gold glitter to truly look angelic.
Some raggedy clothes and a bandana on a stick will make a great hobo. Oversized shoes would look cute and a beat up old hat. For the child that can endure it add a mustache.
The cowboy is also easy if you have a plaid or western shirt and jeans. Add a cowboy hat, a rope lasso and some cowboy boots for an easy quick no sew costume.
The devil needs an outfit of red clothing. A cape can be added; some scary long fingernails and the pitch fork and mask or add some horns to a cap. All can be found at the thrift stores and require very little creativity.
There are a variety of locations for thrift stores in Colorado Springs, a great place to find costume stuff.To name just a few, you can visit Goodwill Industries, ARC, and the Salvation Army.