Oh, the Bay Area.. so, so, so much to see and do! I love it. @janbb has the right idea.. lots to see in Marin and it’s beautiful. You could spend a whole day driving around the Marin Headlands and checking out all the historic military installations and wilderness. Muir Beach isn’t exactly secret, but it’s not as crowded as other beaches. It’s a small hike from the parking lot, but worth it. Mt. Tamilpais is great, and so are Alcatraz and Angel Island.
There’s Napa/Sonoma, which isn’t too far, if you like wine and wine tasting; they also have some very good restaurants in that area. Also, don’t forget the East Bay! Oakland and Berkeley have some really cool areas that are fun to walk through and have lots of shops and restaurants, like College Ave. There’s the Chabot Space and Science Center, they have a huge telescope/observatory and they’re open until 10pm on weekends. There are loads and loads of parks pretty much everywhere in the Bay Area, if that’s your thing.
Tahoe is about 3 hours away from me (in the East Bay), if you’re wanting to take a longer day trip. It’s really beautiful up there, Truckee is a quaint little town with an old-fashioned main street feel.
If you haven’t gone to Santa Cruz yet, definitely go! It’s one of my favorite places on Earth. :) If the weather is nice and the season has started, you can take an open-top steam-powered train from the Boarwalk up into the Santa Cruz mountains, ending up at Roaring Camp in Henry Powell State Park, one of my favorite places to go for peace and nature. There’s a redwood trail walk with some giant redwoods, the San Lorenzo River, and plenty of little places to go down and sit by the river.
If you go to SC, take a drive down to Moss Landing. You can rent kayaks and paddle up the estuary, where you are pretty much guaranteed to see seals, otters, and loads of birds. It’s very peaceful and beautiful.