Muir Woods is the easiest old-growth coast redwood forest to reach from San Francisco, and a half-day tour solves the two hardest parts of getting there: the parking reservation system and the windy drive over Mount Tamalpais. A guided trip gets you in, gives you about 90 minutes on the trails, and folds in a Sausalito stop on the way back.
The walk through Muir Woods is mostly flat boardwalk along Redwood Creek, looping past trees that have been growing since before the city was founded. The trees here are coast redwoods, the tallest species on Earth, with the oldest in the grove pushing 1,000 years. The trail is shaded, cool, and quieter than you expect once you’re past the entrance area.
After the redwoods, most tours stop in Sausalito for a short break with views back across the bay to San Francisco. Hotel pickup is included from most San Francisco hotels, which makes this a clean half-day option without rental car logistics. Wear layers; Marin is reliably 10 to 15 degrees cooler than the city.