I've heard of this elsewhere and thought it would be a cool idea - pick 2 or 3 women from each sorority. Then draw names out of a hat and they each stay over at a different house for the weekend. Or, perhaps the Zetas would be nice enough to host an all-sorority slumber party in their giant basement.
Something else we talked about that never got off the ground: Panhel aerobics, if there is someone in one of the sororities that is a certified instructor.