I like the ideas that KSUViolet posted, we did many of those and they worked well.
Idk if you do a senior farewell (sendoff, etc) but to go off the scrapbooking idea, every sister made a page for each graduating sister, with pictures of them together, notes, memories, etc. It's one of my favorite things that I have and it was really fun to make all the other girls' pages. We often did special nights where everyone would get together, share supplies (a few of us were crazy scrapbookers lol) and talk, watch movies, chill out, order some pizza's whatever
In the fall we did things like apple or pumpkin picking.
If a big movie was coming out (like when the last harry potter came out) a whole bunch of us would go as a group (maybe group rates for this too??) and we'd go get dinner beforehand.
We tried to do a sisterhood dinner everyweek in the student union (and the girls that lived on campus would use their extra meals on the girls that lived off campus or in the house) and wear letters.
In addition to having mixers, we'd do a "thirsty thursday" where it was just us hanging out if we didn't have a mixer, another fave was "taco bell tuesday" or wendy's wednesdays.... i'm seeing a trend toward food....
We'd also do a game night, and do teams for cranium or charades, anything for a big group.
We also tried to do a date party midway through the semester.... We also made our formals pretty big affairs but we also went camping one year, and that was a lot of fun.
I'll try to think of some more
eta: sorry for crashing the sigma forum!!! i didn't realize it until after I posted and thought some of the ideas might still be helpful