I had to come up with an impromptu costume for dh and myself one year. We went as a spider and a bee.
For the spider costume I used a black velvet shirt and black leggings (any black shirt and pants would probably work). Took two pairs of black tights (opaque!)stuffed them and put a wire in them for the spiderlegs. I made a U-shape with the wires, stuck each end into a leg and bent them to form spider legs (I used 2 coat hangers but a lighter weight wire would've been better). Then I attached one pair of tights to each of my sides. I just used a black elastic backbrace I had but you could use a black sash or wide belt. Put the crotches of the tights under the brace (sash or belt) with a leg hanging out over the top and the bottom of the brace on each side. Add a red diamond shape to the costume if you want to make it a black widow.
Bee costume - black sweatshirt and pants (or leotard and tights). Wrap yellow electrical tape around the top part. Buy little cheapo wings and antenna heaband. Or you can make your own pretty easy. Make wings with wire hanger and white pantyhose. Use glitterglue for marks on wings if you want. for antenna just get a plastic headband, pipecleaners, and styrofoam balls for the tips. The best part about this costume is that it's completely reusable, just take the tape off. It's very warm and comfortable too since it's basically just a sweatsuit.
When I was little my friend and I made a great two person costume but this idea could be revised for just a one person.
We were Conjoined Twin Alien Robots. We took a big box and cut a hole on each side for armholes and a hole for each of our heads to pop out the top. We got in and put an arm around each other to get in close and put the other arm out the hole (so we each had an arm sticking out). We took the box and glued bottle tops for knobs and some other gadgetry looking stuff. Spray painted the whole thing and drew on dials and whatnot. We wore green tights (use silver/grey if you want to be a plain robot and not alien). We also stuck egg carton cups under the tights so we looked like we had warty growths all over our legs (don't ask me why, it just seemed to us that aliens might have warty green legs
). Then we took tinfoil and made helmets that tucked into the boxes so our heads and necks were completely covered.
This costume is fun because you have to cooperate in order to walk correctly. It's almost like walking in a three legged race. The kids will have fun practicing their walk all the way up until Halloween!