The San Francisco Bay Area abounds in great locations for photographing cosplayers in various environments. Let's start from the north and work our way south.
Marin Civic Center
This futuristic architecture by Frank Lloyd Wright has been in a couple of science fiction movies. It would be perfect as a backdrop for more sci-fi themed costumes, particularly Star Trek and Star Wars.
Starting around the time of the Civil War and lasting until the middle of the 20th century, various military bunkers were built to help protect the San Francisco Bay from invasion. Today, the Headlands are a national recreation area. The bunkers are scattered around the site. They're made of crumbling concrete and would make good backdrops that suggest a post-apocalyptic world or even medieval castles.
More to come!