Wel, this isn't for a greek org, but a service org I'm in is having a theme party in a few weeks and the theme is "Rolling Through the Decades" and everyone is going to come dressed as his or her favorite decade.
Another traditional party that we have is called Boots and Boxers, where everyone shows up in boxers and some kind of boots. I've never been to that one, so I don't know what goes on at that one.
Hope this helped some
