A little late, but what about a 5k? I know of a few organizations/programs that have used that as a fundraiser in the recent past. I imagine the biggest hurdle would be finding a place to map out the race, depending on how easy the city/town is to work with about those kinds of things, but depending on the resources in your area you might be able to use a school, park, or fairgrounds to help out with that.
I recently participated in one to benefit the local Humane Society and they started at the local fairgrounds, had volunteers map out the course, local businesses donate drinks/snacks for before and after (bagels, coffee, water), and also some coupons from local places to put into a goody bag for participants. They also had t-shirts made, an extra cost but maybe an extra incentive to get people to participate. Get the word out and charge $10-$15 for registration. It would be a lot of work, but maybe not too bad if you had a group of willing parents to put forth the effort.