I help organize groups that are interested in volunteering their time while in San Francisco. Some of these groups are from various schools or churches throughout the United States. The San Francisco Food Bank http://www.sffoodbank.org/ always needs volunteers to sort, package and distribute food for hungry people. I would suggest to anybody that is interested to visit them at 900 Pennsylvania Avenue and speak to Sam Marcum or Lisa Start or ask for their email addresses.
They do amazing work and I suggest you help them out!