have you done park clean ups? community clean ups? that sort of thing? what about tree plantings for reforestation of an area?
you might even have a plant sale or help people in your school start organic gardens in their homes or on their land. I have a great gardening book for container gardening that can manage a huge amount of herbs around a tiny porch (literally, mine is about 6 ft long and 3 feet wide). my herbs are organic. many of my neighbors now have small gardens on their patios too. it's becoming more popular.
you can grow the veggies/herbs in the gardens you already have, and then sell plants to support your organization, and then offer to help them plant such a garden--if it's goign to be a large scale project.
also, a retirement home may do well to have you come and do a garden for them. CUtting gardens are particularly nice, and you'll have to use raised beds so that they're accessable. This is a great environmental community service project that many people will enjoy.
there's a group in philly called the green guerillas. they do "overnite" community gardens in depressed neighborhoods and empty lots. Then, they maintain the gardens and teach the community to maintain the gardens. It's all on donation. It's really cool. it's like "surprise garden" or something.
You could just join your local community garden project and try to build as many as possible throughout your community. See what happens.