Muir Woods
A fine day out without going too far. Make a reservation.
I have a love/ hate relationship with Muir Woods. If you are only in SF for a short time or you have mobility issues, then it is a great way to see the famous, huge coastal redwoods. I think Muir Woods is a bit of a “redwoods zoo” though — accessible, with a level boardwalk and you can walk the whole location in an hour or so. And the trees are really amazing if you have never seen them before.
You need to plan to take most of a day and be sure to reserve a parking spot ahead of time or schedule a shuttle ride (the shuttle is not operating during the coronavirus pandemic): https://gomuirwoods.com and expect crowds. There is NO cell phone service or WiFi at or around Muir Woods National Monument so download your parking reservation or shuttle ticket in advance.
There are many other choices for redwoods hiking outside of the City where you will only occasionally pass another hiker. They are all a bit farther out than Muir Woods and require sturdy shoes, sunscreen and water, but you will get the real redwood forest experience.
For more challenging hikes I absolutely love: Henry Cowell Redwoods State Park, Big Basin Redwoods State Park, Purisima Creek Redwoods Open Space Preserve. I’ve never been to Humboldt Redwoods State Park because it is about a 4 hour drive north of the city, but if your itinerary takes you up that way, I would definitely recommend it. See if you can fit Mendocino into the trip as well.
And for just a wonderful NorCal hike, keep in mind Mt. Tamalpais. It’s a great excuse to drive up the Pacific Coast Highway, visit Stinson Beach and hike Mt. Tam trails. Better than taking the kids on a wine tasting trip to Napa!