The best part is Ridge Road on the top of Mt. Tam. It's a rolling hill road with spectacular views of the Bay Area: San Francisco, the Bay, the Ocean, the Tamalpais watershed. Then you get a swurvy, switch-back descent through some redwoods to Alpine Dam. You then climb out of the Tamalpais watershed into Fairfax.
It should be noted that from the beginning of the Ridge Road until you hit Fairfax, there's not much in the way of bathrooms or water.