Cool ones we used to have (or have had) are:
"War Games" with the Pikes. We had 2 days of running around on campus with water guns soaking each other and getting points for "kills". You have to make sure your members don't get out of hand though and use them inside buildings, because we are no longer allowed to have this mixer.
"DreamLand" mixer with the TKEs this year. We had a pajama mixer where we played Twister and had s'mores, and told about our most embarassing moments (there were some classic ones!). After that, we went to one of the brothers houses and they had an entire room completely filled with white balloons and we whacked each other with the "clouds" for at least 2 hours. I have never been so tired after a mixer!!!
A funny as hell one that I've heard an idea for is a "Pirate Mixer". Granted, prissy girls won't like it, but I sure as heck would have fun. You split everyone up into a few big rafts, bring water guns and water balloons. Then you just bomb and shoot the hell out of each other! Of course, everyone has to be able to swim, or you do it in a really big swimming pool, but I think it would be totally awesome!