Halloween should be just as much fun for adults as it is for kids. I believe one of the things preventing adults from having grown up Halloween parties is the idea that it has to be some sort of big production. People look at the pages of Martha Stewart magazine filled with perfectly handcrafted items and homes decorated like elaborate stage sets and they think "I don't have time for that!" Don't let your fear of falling short of perfection scare you away from having a great time on Halloween.
Have fun on Halloween by hosting something simple like a movie night. Pick a Halloween themed movie, preferably something PG, that is fun and light in tone. I once attended a Halloween party where the main entertainment was a graphic slasher flick that killed my enthusiasm for the party almost as fast as the bad guy was killing off second rate actors. A party is supposed to make guests loosen up, not lose their appetite.
It's pretty easy to build a general theme around a Halloween themed movie like Ghostbusters. Since the theme of the movie revolves around ghosts, you can ask guests on the invitation to come as the ghost of their favorite dead person. The dead person can be someone famous, or an eccentric deceased relative that provides plenty of irreverent inspiration. Many people don't like to bother dressing up in costumes for Halloween. To make everyone feel welcome with our without a costume while still sticking to the spirit of the party you can encourage guests to dress like mourners and wear a lot of black to the party or wear all white for a ghostly effect if they aren't interested in wearing a costume.
If you feel the need to decorate, throw solid white colored sheets over the couch and chairs that guests will be sitting on. Turn the lights down a little if you have dimmers. If you happen to have a solid red tablecloth, maybe left over from Christmas, that would be perfect for decorating whatever you are using as a buffet table.
For snacks set up a popcorn buffet. Mix a large batch of plain or lightly salted popcorn with a sweet candy like M&Ms in one larg bowl. Mix another large batch of plain popcorn with chex mix or some sort of trail mix for a saltier take on popcorn and then mix up a third batch in a third bown of popcorn and another type of candy, maybe gummy worms. Even if you mix candy and popcorn equally, it's still a lot cheaper than serving popcorn by itself. For drinks you can serve soda pop and regular tea. You don't want an overly flavorful tea that is going to clash with the popcorn.
If you are concerned about the party dying when the movie isn't on, buy a few halloween themed games, like Halloween bingo, at your local toy store to bring your party back from the dead.