Unfortunately, sometimes it's easier to write a check. Particularly young people, who are more volunteer-inclined than any generation in U.S. history. However, there are many organizations, including Youth Service America, who would be happy to have your check. There is no substitute, financial or otherwise, for the power of human intervention in meeting the critical needs of the community. So I would encourage you to call an organization like DC Cares, even if it's only monthly or quarterly.
Intent is more important than quantity.
I am her mentor and am trying to figure out what she should do to meet her 36-hour volunteering requirement for school. For kids who are shy, local humane organizations are great places for them to volunteer. Dogs and cats need to be washed, pens need to be cleaned and puppies and kitties need to be played with. I would also suggest participating in a parks and recreation program that might do site beautification in the spring and summer. And there are currently 12,000 opportunities available to the American public.