What a great question! When I was in Girl Scouts, my troop did our silver award project and all the other requirements together. It made it so much easier for everyone.
I assume you're looking for ideas for the actual project? My first suggestion would be to brainstorm with the girls to see what ideas they have. If the girls can come up the project idea themselves, that might be best. It's really inportant to have ALL the girls on board with the project; otherwise you will have some girls won't be fully comitted to the project.
You do the brainstorming a few different ways. You could ask the girls what they are interested in, what they see the needs in the community to be, and maybe even have someone from the city to come and talk to them about what social services are provided. Maybe you could do some field trips to visit different places in the community to see what ideas the girls come up with.
Encourage them to think creatively. If they all like animals, then perhaps a project at the local zoo is the thing. If the common interest is nature, then perhaps work at a nature conservatory is the way to go. If the girls want to do something for children or the elderly, then perhaps a project at a local community center or a retirement center might be up their alley.
My silver award project was putting in a garden at a local nursing home that was accessable for the clients who were in wheelchairs. We got Lasrge beer barrels, cut them in half, treated them with verathane, and planted flowers, herbs and vegetables. Once we got everything established, the nursing home took the processover. The garden is still being used today, 25 years later.
I hope this helps. Good luck, and have fun!