Muir Woods National Monument preserves one of the last remaining old-growth coastal redwood forests in the San Francisco Bay Area, and it is extraordinarily beautiful. The ancient trees rise 200-plus feet from a cool, dim forest floor; Cathedral Grove has specimens over a thousand years old. The main valley trail is flat, easy walking, and can be done in about an hour, though side trails climb steeply into the surrounding hills.
The main warning: Muir Woods is extremely popular, and the parking situation is severely limited. Reservations are now required to park at the monument or at the nearby Muir Beach Overflow lot — book well in advance on the Recreation.gov website. Alternatively, take the Muir Woods shuttle from Marin City or Sausalito on weekends and holidays; this is actually the more relaxing option. Weekday mornings in spring or fall give you the best chance of genuine quiet among the trees.