I know people hate her, but Martha “Antichrist” Stewart does have some neat Halloween ideas in the holiday section on the http://www.marthastewart.com website.
Other things I have done:
Fill kids’ trick-or-treat plastic pumpkins with candy (ideally the candy of your childhood, not this new-fangled candy) and use as centerpieces or anchoring the buffet table.
Elaborate, but effective: Make tombstones out of cardboard and put goofy directions on them, so arriving guests follow them out to the back of your house, or wherever you want them.
Fill rubber gloves with ice and freeze, then peel off gloves and float the ice hands in your punchbowl.
If your guests like activities, have a pumpkin painting contest by stocking up on smallish sized pumpkins and poster paint. I’m too paranoid to have people with carving knives around alcohol.
Totally messy and designed for kids, but my (drunk) friends liked it too: Toilet paper mummy game – have teams wrap one member with toilet paper to be a mummy, first/best mummy wins.
If you are artistic: Cover your upstairs windows with white paper, and then add a black cardboard cut-out of life size monsters – Dracula, Frankenstein, Wolfman, etc. After it gets dark, turn on the lights in the rooms, and from the backyard you will just see the silouettes of the monsters who have taken up residence in your home. Instead of monsters, you could also do little bats, ghosts, or spiderwebs. It helps if you put a lamp directly behind the window, as the overhead lighting might not be bright enough to make a sharp sillouette. You can also play around with using colored paper or colored light bulbs.
One of the best things I ever saw at a Halloween party was a guy who found about ten fake black birds, and set them up in a row on a small section of his fence, and on the peak of his roof, a la The Birds. Everyone did a double take when they first noticed them.
