There are many commercial costumes available at the local stores, however between the cost, and lack of imagination, and the fact you end up with a half dozen kids all looking the same at a party. Nothing beats homemade. Below is some basic ideas for you to play with. All are cheap, easy, and allow for tons of imagination.
Do it in tights & leotards or Sweats
Skunk Black with white strip down the pack.. be careful everyone may run
Bunny Pink, brown, or white, add a powder puff or large pom-pom to the backside, and a set of ears make from constriction paper and a plastic headband. Now just hop on out and enjoy
Turtle Green with a piece of poster board attached to the back. Simple cut the poster board into a large oval, make random marks like a turtle shell, and paint.
Puppy Brown, tan or white, either paint, or pin paper spots on. Make floppy ears and bobby pin to hair over your ears. The tale can be most anything, depending on what type of doggie you wish to be. Add a little make-up.
Kitten Gold, brown, tan, white. Make ears from construction paper, attach to head band or bobby pin in to hair. Sew a simple round tale, and pin on.
Spider Black, For the legs grab some cheap black hose, stuff with any thing dark and light weight. Either sew, or staple to keep stuffing from falling out. Pin to clothing, add a little make-up
Costumes from the closet or second-hand store
Ghost: Grab a old white sheet, cut out the eyes. Be sure its is short enough not to trip you. Take scrap piece of sheet, and sew up a simple treat bag. OR Why be the same old plain ghost? A ghost going out in public might like to get dressed up a little. Use the same old white sheet, cut out holes for the eyes and a small slit for the mouth. Then dress it up a little. Put eye shadow and lashes around the eyes. Lipstick for the mouth. Put a little color on the cheeks. How about earrings and a necklace ? You could color in a beard or mustache. Use something besides just a white sheet to be a little more fancy
Clown: Sweet clothes that are too large. Decorate by using either fabric paint, or markers. For those who dare try tie-die. Tack on some large pop-poms, add a cheap wig, hat, and make-up. You may also stuff the extra space in clothing with tissue. However this could be messy, if you need to make a "pit-stop".
Scarecrow: Over sized flannel shirt, faded blue jeans with patches, rope for belt and to tie at the cuffs of his jeans and sleeves, stuff the shirt and jeans with plastic grocery, or paper, a little bit of straw to hang out of jeans and shirt. Add a little bit of make-up.
The American tourist: Guys loud Hawaii shirt, shorts (if weather permits), Large straw hat. Gals bright loud dress, flats, big sun hat, straw bag. Don't forget to have a camera, and maps hanging out, and that look of being lost. Guys can use a tourist bag, (like you get from travel agents, and gals can use the straw bag for their treats.