There's an abandoned school between China Camp and Mcinnes Co Park. A lot of open space, but there is some stuff that could be a lot of fun.
Here's another photo of that sculpture, with some people for scale.
I got up and climbed on it and the surfaces are actually pretty sticky. The jungle gym equipment is all completely solid too. We took a peek inside, but didn't go in. I wouldn't recommend it, who knows what's been living in there.
McPhail School, San Rafael, CA 94903