I like the ideas I see posted - I think I've heard the Kappa-soup one before.
A related idea is a Cansome-Ransom. I'm stealing this idea from a local GLO on our campus but it's very clever - they "kidnap" people from other organizations, greek and non-greek (one year they kidnapped the "RA" from a floor) and to bail the person out your organization has to cough up 100 cans of food to be donated to the local soup kitchen. The "kidnappies" watch movies, get fed pizza, and have a great time while in jail. Fun idea I think, and really low-effort for high reward to the charity.
Another fun idea is to make being active in the community part of the philanthropy. One year we helped a local charity sell a dozen flowers except 6 flowers went to the designated person, and 6 went to a local nursing home delivered by a sister (or to a cancer ward at the hospital, etc). Furthermore, all profits went back to the charity organization. It was a great idea, if we had the woman-power we would have tried to do it completely ourselves. (you need lots of "deliverers") You can look to the cancer society's daffodil days as a bit of a model - although this is a twist on that program.
My chapter (Delta Xi) does a thing called quarter-mile-of quarters, where organizations collect quarters for the children's hospital, they also rent inflatable games and face painters and have a whole festival on campus on the day when they "count" the quarters. But that is way more complex than I can explain on this board
) It is very cool though - the whole campus loves it.
Best of luck with ideas!!