Muir Woods National Monument is located a few miles north of San Francisco past the Golden Gate bridge. Named after John Muir, the famous naturalist, this place is home to old Redwood trees that are hundreds of years old. Some trees are about 800 years old. There are 6 trails of varying lengths and levels of difficulty to walk around and explore this wonderful area. Muir Woods is open 365 days of the year. The entrance fee is $7 and is expected to to up to $10.
Wear good, walking shoes and remember to carry a jacket with you. The road leading up to Muir Woods is narrow, and do watch out for bikers. The park gets awfully crowded during holidays and weekends and finding a parking spot can be difficult. Plan to spend at least half a day exploring the place. If you are in a hurry, a couple of hours will also do. If you have the time you can go up to Mount Tampalpais (Tam) from Muir Woods. The road is narrow and steep in some places as you wind your way up the road. Once on Mount Tam, there are a couple of trails to walk around and explore. There is a trail from where you get a splendid view of San Francisco city and Marin county. If you are there towards late afternoon you might even see the fog roll in from the Pacific Ocean and envelope the whole city.
And if you have the time and inclination you can grab a cup of coffee in downtown Mill Valley.