Skyline to sea trail in West Waddell Creek (starting down from the ocean access) or redwood hikes in Big Basin Redwoods State Park. In the north bay, I enjoyed Joaquin Miller State Park/Redwood Regional Park, but it's probably not worth the drive if you can just head to the coast for similar hiking.
Purisma Creek Redwoods OSP on the Peninsula is my go to. You can go as small as a leisurely 3 mile hike through lush redwood forest or gradually move up to an 18 mile conditioning loop with 4800’ of elevation gain. Uvas canyon county park in Morgan hill has a good mix of moderate creek side hiking on Contour trail and steep conditioning on knibbs knob. Weekend day use parking requires an online reservation. In the north bay I enjoy Steep Ravine from the Marin coast to the summit of Mt Tamalpais or Colier Springs Trail from Lake Lagunitas.
Greetings from NorthBay! If you head this way, some of my favorites on Mt. Tam and in Point Reyes are:
Baltimore Canyon in Larkspur - redwoods and waterfalls, walk to town afterwards.
Blithedale Ridge in Corte Madera - amazing views, lots of offshoot trails to explore Tam
Dipsea Trail in Mill Valley - same as above, takes you out to Stinson Beach, shuttle back.
Rodeo Beach in Sausalito - picnic on the beach then go wander the hills and take in the sights before heading into town for refreshments
Tomales Point - elk watching, kayaking nearby, stop in Point Reyes Station to pick up picnic goodies on the way (Toby Feed Barn) or stop for oysters on the way home at the place du jour on the corner of Sir Francis Drake & Rt 1