Ok, I don't know how big your pledge class is or how craft you are...but what might be neat is to get t-shirts of all kinds: Your bid day shirts, party shirts, shirts that represent your sisters activities, etc... and make a quilt for them! They could put it out in your chapter rec room or hang it up..whatever! It would be something that all of them could look at and sisters for years to come would enjoy it!
Good luck with your project! It sounds like a lot of fun!