Oh my gosh, Optimist, that party idea is so funny. Granted, I would never use it for my sorority, but I ran into the other room because I HAD to tell my housemates about it.
Some other ideas are a graffiti party, foam party, 1950's/60's/70's/80's themed party, hayride/western party, beach party, heaven and hell party...there are so many different themes you could choose. I'd also suggest looking in Greek t-shirt catalogs and websites to see what other groups have done. You can get A LOT of ideas from looking through those.