here's some ideas:
Buy and Elvis wig, some gold trim. Then get an old sheet, and make a traditional ghost costume. Combine the two and go as the "Ghost of Elvis."
Now, this may offend, but dress up as in a white short-sleeved dress shirt, black tie, black pants, with white socks and a backpack. A 10 speed, a bible, and some pamphlets would round out the costume. I think you get the idea....I did this last year, and won the contest at a party.
I am going to a wedding, poorly timed to have it's reception on Saturday Hallow's Eve. Thus, me and my buddies are going to be dressed in suits. I had the idea (after watching "Kill Bill") of just buying some samurai swords and black masks and going as the "Crazy 88's" from the movie. We won't even have to change, and it's a pretty cool costume. We have a girl dressing up in a yellow motorcycle outfit like Uma Thurman did in the movie.