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We used to have Math-querade parties, just to make use of a good pun. Costumes were optional, but plenty of us dressed up anyway.
Sometimes we'd mak math-shaped sugar-cookies -- just make a batch of sugar-cookie dough and cut it in the shape of various operators and greek letters (then have fun decorating with icing). It's a fair amount of work, but you could even turn it into a party-community activity (have the dough ready ahead of time, then get everyone to roll out a bit of dough and start cutting out math shapes).
It was always good to have games around. Encourage people to bring their own -- they're likely to have many. If you know any math profs well, see if they'll loan you their games, or suggest some (about 1/3 of math profs I know *loved* puzzle-games). Any game with deduction is usually a sure-fire hit, including various card games or round-the-table games like "Mafia", e.g. http://en.wikipedia.org/wiki/Mafia_(game) [yes, those parens are part of the url, it seems).
Alcohol is fine, but recognize that not all geeks like drinking, and that is fine. I'd suggest accommodating moderate drinking, but skip the hard alcohol.
Unless the math students/geeks you know are assholes, any effort you make with the intention of having fun will be warmly-received.