If someone in your group of friends has a distinctive way of dress, you can go as him. I did that junior year of high school, and it went over well. (I stole the original idea from my friend Paul, who went as someone else a couple years before.)
–Cliffy
I dressed up as a ghoul a couple of times, spoke in a generic horror movie accent and called myself Riggo Mortis. I already had the big black shoes, black jeans, and black sweater. Spent about $25 on a tin of Halloween facepaint and a black hood.
First year I dressed as Riggo, I won a prize for being the scariest one at the party.
These are fantastic…
Wear normal clothes covered in large amounts of cotton, shredded felt, and dryer lint. Wear big floppy rabbit ears. You are a dust bunny.
(Sorry if somebody already mentioned the next one. Searching the thread turned up nothing)
Wear all black clothes, except for a white circle on your chest with and 8 in it. Confine your speech to 20 responses such as “Sources say yes” and “Ask again later”.
I was Wyoming once.
Plain, rectangular cardboard box (you’re already halfway there).
Cut the key & other extra stuff from Wyoming map.
Apply Wyoming map to front of box with clear Contact paper.
Find Old Faithful on Wyoming map; punch small hole through map there.
Cut out holes for head, arms, body, etc…
Put plant sprayer in box with you.
Once every 60 minutes spray crowd through the Old Faithful openiing.
I was also a giant kelp at a Halloween party at which the hosts had a hot tub - shredded thick green/black lawn leaf bags applied liberally to head and body worked just fine, and it floated along the top of the hot tub nicely.
I guess being a mailbox for Halloween as a kid scarred me for life. 
Cut a leg off a pair of old brown pants. Seal one end so that it tapers to a point. Stuff it and wear it dangling from the back of your pants. Wear all brown clothing. Get a pair of fangs to fit over your upper canines. Get an old hat and glue as many pennies on it as you can. Speak in exaggerated sibilants. You are a copperhead.
Glue cigarette butts, egg shells and other debris all over your shirt and go as a mess.
Considering that was the night teenagers mowed down mailboxes around here, I would think so. They didn’t get too far once they ripped out their oil pan on the wrought iron.
That’s brilliant! I already started my costume for this Halloween, but I’m so doing this next year! I love the Magic 8 ball idea, too.
The SO did this my freshman year in college. We were invited to go trick-or-treating with some people in my dorm and we didn’t have much in my room to get dressed with. But in 10 minutes, the SO went outside of my room dressed and ready to go.
He was a bit scruffy, hair in shambles, with sweatpants, a shirt with stains and two different shoes.
He was ‘late for a midterm’. 
Get a large enough box for you to wear, cover it in Xmas paper and go as a present.
Get a clear shower curtain. Get a large sheet of cardboard and cut a disk about the size of a trash can lid. Make the curtain into a tube with some clear packing tape. Using duct tape, seal one end of the tube with the cardboard so that the whole thing looks like a jar. Buy a cheap cat mask and walk around wearing the jar. You are a Bonsai Kitty.
Thought I’d bump this to tell everyone what my costume was. I ended up dabbing some fake blood in the corner of my mouth and coughing daintily into a blood-spotted handkerchief every minute or so. Presto, I had tuberculosis!
At a party full of much more elaborate costumes, my gimmick was a big hit. A friend of mine said that the only thing that could have made it better would have been if I had brought five or six other blood-spotted handkerchiefs to pass out furtively amongst partygoers – who would themselves then start coughing as the party went on. The whole thing was a lot of fun.
Thanks to everyone for all their creative suggestions!
For my costume, I wore a business suit and some blue gloves so I could be one of the Hands of Blue guys from Firefly. I even had a couple of blue glow-sticks taped end-to-end to make a hemorage stick device like the one they use on the show.
Later on, we figured out that if I took off the jacket and proceeded to use a camera while wearing the gloves, I could become CSI Guy.