Before San Francisco became "San Francisco" it was called "Yerba Buena" - Spanish for "good herb".
Yerba Buena Gardens is mostly herb-free these days, but it does offer grassy picnicking opportunities in Downtown! Covering two blocks, the gardens also house a Martin Luther King Jr. Memorial, Zeum, Yerba Buena Center for the Arts, an ice rink and a bowling alley. The MOMA, Contemporary Jewish Museum and Metreon adjoin.
Website: http://www.yerbabuenagardens.com/
Wikipedia: http://en.wikipedia.org/wiki/Yerba_Buena_Gardens