I can’t think of any open space on the peninsula, east bay, or South Bay that is off limits to public use. The only lands down here closed to the public are city owned watersheds which is utterly ridiculous. East Bay MUD land is open to public use and there are no problems. There is some irrational fear in SF and SJ that people will harm the watershed.
Santa Clara Open Space Authority has three properties open to the public. I hope they open some of the others, like Blair Ranch and Doan Ranch soon. They also have property that would allow a hiker to go from Almaden Quicksilver County Park to hike into Calero County Park and Ranch Canada del Oro Open Space. That would be excellent. The property they bought from United Technologies is supposed to open as Coyote Ridge OSP IN 2018. They only bough it in 2018, which since they bought it in 2015, doesn't seem like turnaround.
I have hiked at Blair Ranch and Doan Ranch on one-off says that they opened it to the public.
Doan Ranch is East if 101, and rolling golden hills.
Blair Ranch is right next to Rancho Canada. It has some really steep grades, it would make a nice addition to the Rancho Canada/Calero County Park complex, especially if they connect it to Almaden Quicksilver.
Actually, the opposite is probably true. Allowing hikers a wide swath means less illegal activity on public land. Specifically, less dumping and growing illegal substances.
Cotoni-Coast Dairies is a 5,800 acre property that POST donated to the BLM, and Obama made it a national monument before he left office.
San Vincente Redwoods is 8,500 acres between Davenport and Big Basin. It has 70 miles of roads and trails. I'm not sure when it will open, or what it will be under.