Also if you go to Muir Woods, get there early. Parking can be a challenge. I didn't have any parking problems at Armsttong.

Redwoods and sequoias are two different things. Redwoods you can find along the coast, they are the tallest ones. Sequoias are in the Sierra mountains, they are tall but also the biggest around. Both are impressive! You can see good redwoods in Muir Woods and also it's near Muir Beach, Stinson Beach and the Pelican Inn, a nice day trip from SFO. Other people have noted there are great redwood state parks up in the Sonoma area and down near Santa Cruz & Big Sur. For big sequoias, closest place is prob. the Mariposa Grove, which is in Yosemite.
Place to see the TALLEST redwoods is about a day's drive north, in a particular grove in Redwood National Park, or more accessible grove is Humboldt redwoods state park off 101.

Armstrong State Park in Guerneville -- maybe 1.5 - 2 hours north of the city in Sonoma County.

Guerneville is great. The Redwood park is really amazing, and the town is off the beaten path so it's pretty accessible. Great food, low key accommodations, and it's at the bottom of the Russian River Valley, so the wine touring/tasting is terrific (esp. if you like Pinot).

Don't miss Boon eat + drink, or the taco truck in the Safeway parking lot.

Just check and make sure it's not during the Bear festival, when the town turns into something different than you may expect.

Check out Muir Woods. It is right outside SF.

Armstrong State Park in Guerneville -- maybe 1.5 - 2 hours north of the city in Sonoma County.

We'll be in San Francisco in August and will have time to explore the area outside of the city. I think our outdoorsy middle schoolers would like to hike in the area and I'd like to see the huge Redwood/Sequoia trees. But there are so many options, it's overwhelming. We would have at least 5 days outside of San Francisco. Any suggestions on which parks to go to and where to stay?