I plan most of the parties for my department, and I have to second what Scrbblchick said: alcohol is a bad, bad idea. People have gotten fired because they got drunk at an office party and did something completely out of line. People can do stupid things when they're sober, obviously, but the kind of stuff that gets you fired almost always stems from too much alcohol.
I usually plan things during the day so people can come and go. I get snacks and sweets and we may have a get-together in the afternoon. That seems to work best for us. It may not for every business, certainly, but it does well for our environment.