Our mixers had to be dry, no bones about it. But believe it or not you can have fun without alcohol.
We had food fights on one of the quads, ice skating, bonfires, movie nights (all you need for that is someone's house with a large living room), cook outs at baseball/soccer games, etc. Think about activities you can do outside or in a private home with cheap supplies...building a fire is cheap, hotdogs and burgers are cheap, renting a dvd and projector is cheap, etc.