Moving south, it's hard to miss the city of San Francisco, the legendary city by the bay.
Off the top of my head, however, there's nothing I can think of specifically that would be good for cosplay photographic backdrops... How about this?
Palace of Fine Arts
This structure survives from the 1915 Panama-Pacific Exposition and is located next the Exploratorium museum. Architechturally it's got Graeco-Roman columns and a sort of ancient look to it.
Sunol Water Temple
This is another structure with a very Graeco-Roman vibe to it. It's more remote than the Palace of Fine Arts, which may be a good thing or a bad thing. If you need privacy, or if you're filming a movie, I would suggest Sunol. If convenience and a quick snap-shot is all you need, then by all means go to the Palace of Fine Arts and check out the Exploratorium in the same day.
San Francisco's Golden Gate Park abounds in all kinds of structures and environments. Perhaps I will touch on this later.
Castle Rock State Park
Going a bit south and converting to a naturalistic landscape, Castle Rock is popular with hikers and climbers. There are beautiful woods, craggy cliffs and caves as well. This is where I shot one of my fan films, although the State Park system prefers that if you show up with lots of filmmaking equipment, and look like a large-scale studio, they will probably want to charge you some money and make you sign permits. But for low-profile missions and photo shoots, they should be okay.
Further to the south, Hakone Gardens simulates the look of old Japan. Several pieces of authentic-looking architecture surround a koi pond, and the grounds are landscaped by all manner of Japanese plants, including some really big bamboo. Pretty darned good for anything with a samurai theme.
In a pinch, some locations in San Francisco could be used, such as Japantown, and the Japanese tea Garden in Golden Gate Park.
With all of these, please check for availability and permission first before you go!