I was actually also just elected as my colony's (Delta Phi @ NYU, go us!!!) Philanthropy Chair. We were a local sorority for 23 years. In recent years we implimented a annual charity event for the American Cancer Society. We host a barbeque for all the students on campus. We get lots of co-sponsers from the other Greeks on campus, as well as some university organizations. It's a lot of fun, we get to use an actual bbq and have carnival type games and stuff. Last year we even had a cotton candy maker! It was great! I am actually the person who invented it on my campus, so if you have questions, please ask!
The whole Greek communty also hosts a Special Olympics basketball tournament, and participates in a NYC-wide event called New York Cares Day.
I have some other ideas too if you want...