Muir Woods is located just north of Sausalito across the Golden Gate Bridge, just a few miles from San Francisco. It is about as different from San Francisco as you can get. The trees here are the giant redwood variety, which grow extremely tall with wide trunks, although they are not as big and impressive here as at Mariposa Grove in Yosemite National Park.
If you are in the area, it is well worth the short trip, but get here early as the car parks are small and fill very quickly. Overflow car parks are available but are 1 to 2 miles from the woods.
There is a small fee to enter the woods; I think it was about $3 each. You then get a guide leaflet with details of the different walks available within the park, with their distances and exertion levels, etc. There are several walks on the main track that won’t get you too exhausted, but if you want to test your agility, there are longer and more strenuous walks. We started off by taking one of the more taxing walks, but after a couple of miles we came across a lot of workers trimming back the trees and were advised not to continue as they couldn’t guarantee we would get back later. We then turned around and made our way back, and once into the main area took one of the easier walks. There were quite a lot of people along this walk--we hadn’t encountered anyone on the first walk. But it was very relaxed and peaceful with the sun streaming through the treetops.
There are lots of information points along the route, so you'll know which trees and plants are what. Guides are also on hand should you want a guided tour.
A rest area in the park has toilet facilities and a restaurant, which sells snack-type things such as rolls, hot dogs, ice creams, and drinks. There is also a souvenir shop selling the usual things--ashtrays made out of redwood trees, books, etc.
You probably will not need to spend a whole day here, unless you want to take advantage of the hiking walks available. You can therefore do Muir Woods in the morning and go somewhere else in the afternoon. Due to the parking situation, I would not recommend doing it the other way round.