Here are some ideas for mixers that my chapter has done...
*Rockstars and Groupies - everyone dressed up then we went to club for karaoke night.
*70s mixer - rented a disco bus then we all went to a skating rink... this was my favorite one, everyone had so much fun!
*I'm Glad I'm Not... - pretty self explanatory, just dress up as something or someone ur glad ur not
*Corporate Lives and Trophy Wives
*Teachers and Students
*Secs and Execs (secretaries and Executives)
*Who wants to marry a (insert name of fraternity here)
Hope that Helps!