Jennifer read about Mt Umunhum, which has an access road I had driven by before on spirited drives, but didn't actually know it was there. We went to check it out and the drive there and up was already part of the fun :)
Once at the top, there is a small visitor center. It used to be a radar site to detect invasions, and was eventually shut down and left to rot. Eventually it was re-habilitated close to its original aspect and turned into a small park with nice views.
The 4 peaks of the bay area, Mt Umunhum, Mt Hamilton, Mt Diablo, and Mt Tamalpais.
The park is actually a very short walk, but the viewpoint is good on a clear day. There are a few signs you can read and an audioguide you can get on android:
Mt Hamilton across the bay
Old radar building that was used to detect potential enemy planes coming from the pacific
Respect to the native americans who used to live here