When I was an undergrad, I was in the largest fraternity on campus. At our largest, about 68-72, the largest sorority was just hitting the 30-35 mark. So, to balance out the sausage fest (and following 33girl's advice), that sorority actually invited one of their nearby chapters to mix with us as well.
By the way, the event we did that night, which I got from GC, was $5 Prom.
There's plenty you can do, though, outside of formals: bonfires, grill outs, outdoor games or lawn game Olympics, trip to the beach or an apple orchard or whatever is norm around you, and a billion different mixer themes.
Seriously, this thread is a gem for ideas:
http://www.greekchat.com/gcforums/sh...ht=Mixer+ideas
ETA: If you don't have a fantastic relationship with any of the sororities, try not to plan an event where you can't force any mixing. For example, volleyball is good because you can mix the teams. Bonfires, though, may not go as well because everyone will stick to themselves if no one knows anyone else.
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