Ohh..sounds like you need to have an in-the-house sister event! We had one that stands out in my mind, and it's a great pledge mixer! The house is divided up into groups, and each group takes a turn doing an activity. We had cup and banner decorating. Each group also had to come up with some kind of song/skit to show to the house, and then each group took turns coming up with a story that had to do with Pi Phi. (It was one of those add a sentence to another sentence to make a story things.) You could always buy some pizza and watch a movie afterwards.
My chapter is also really big on doing game shows. Unfortunately, this isn't something everyone can participate in, but it's also fun to watch! We've done Family Feud and a few others in the past. You could have the seniors and the pledges as their own separate teams. It would be a great bonding experience for the pledges!
Just my two cents.