Besides the usual movie nights, sleepovers, dinners and the like...
If it's nice weather, you could do a BBQ and/or Game Day (bean bag toss, soccer, kickball) and maybe pair up with another organization for some friendly competition.
We did simple things like cupcake decorating, baking together(we had a lot of baking therapy days), and awesome things like pizza-making at a local pizza joint. We also did Zumba a few times and Hot Yoga once, which tied in nicely with our Wellness Pearl, scavenger hunts and had board game nights.
I didn't get to go

but one night had a hayride, campfire and made S'mores. Everyone had a LOT of fun doing that.
Really anything can be made into a sisterhood event. We liked to go to different on-campus events together and then do something separately afterwards.
If an organization on campus is hosting a big philanthropy event, that can also be made into a sisterhood event. Something as simple as watching the game on TV together with some snacks can turn out really fun. You don't need a big budget