Sometimes we have had that problem in the past because we don't have much of an alumni base (our school is in a VERY small town in Texas). ZTA also wins the sisterhood award EVERY YEAR. So, this means that we are awesome with coming up with sisterhoods because we have NOTHING to do but go to school. Some of the things we've done are: go to the zoo, go to houston for an astros game, have a picnic, have a bbq, scavenger hunts, making pledge class videos of any goofy topic, giving each pledge class a bag of items and have them act something out with them, aerobics sisterhoods are the BEST ( I lead one), watching reality shows every week (BACHELOR, Outback Jack, etc), bowling, decorating gifts for philanthropy or fraternities (during greek week, goodluck on rush, etc), shaving cream fights, water balloon fights, service projects (the fun ones like going to the animal shelter to walk dogs), pledge class dinner or dinners to a specific place on a specific night, breakfast mornings, going to other frat/sorority's philanthropy events (also good PR), TCBY, ice cream, coffee, renting videos, getting together to work on big/lil crafts
I hope this gives you some ideas!