As a community service/helping take care of campus sort of event, you could check with the school for help that needs to be done for the school-at-large and organize the chapter to do it (whether it's helping to plant flowers, do some painting upkeep, etc). Events like this might be a bit of a chore, but it also gives a great impression that you're a chapter that helps take care of the college as a whole.
Themed parties can be good choices, too-- our largest success was an 80's party (as outrageous as people wanted to be, clothing-wise). Great photos, lots of fun, great memories.
The key for social activities is organizing events that will help keep conversation going so that potential members can actually talk and meet with your active sisters. Movies nights and such are fun, but they're not really great for meeting folks since it's not exactly good behavior to talk over a movie-- unless you're watching Rocky Horror, of course.
Smaller, local charity drives also are great ideas as AlethiaSi mentioned-- participate in the Walk for Hunger (or similar) if your community has one. Talk to a local movie theater or popular restaurant for your college, purchase a gift certificate, and do a raffle/sale to raise money for a local organization in need-- a homeless shelter, or maybe an after-school program for needy kids.
Good PR for any GLO comes from being visible, being active, and being a group that blends fun and commitment together.
~ Mel.