The bulk of Mt Tamalpais' natural areas are managed by the water district. The five reservoirs provide water to Marin County residents, and are beautiful for walking or cycling. Phoenix Lake in the town of Ross is the easiest to access but the tiny parking lot doesn't accommodate visitor numbers; you could wait up to an hour for a space. Lagunitas Lake (up Bolinas Rd from Fairfax) is equally lovely but its $8 parking fee means there's always a space.
Kent, Bon Tempe and Alpine lakes are all more remote.