During Homecoming week, my chapter spent almost all our free time working on our float. Prior to Homecoming week, I felt like I hardly knew any of these people that I call my sisters. At the end of the week, I felt a very close kinship with all of them. Of course, I'm not suggesting you build a float, but some sort of challenging activity that involves teamwork and problem-solving would probably help tremendously. I'm a little brain-dead right now, so I can't think of any activities, but maybe someone else can come up with something!
Good luck!