Civic Center Plaza is a park square bordered by San Francisco City Hall, the Asian Art Museum, the SF Public Library Main Branch, Bill Graham Civic Auditorium and other government buildings. There's a playground here, and it's nice to lounge about, but there are often homeless people around too. During the day, there are quite enough people around to make it safe. I recommend getting a cheap banh mi sandwich from Saigon Sandwiches (up the street on Larkin) and bringing it here to eat!