I'm always going on day trips with my family. Sonoma has train town with a nice long enough miniature train ride, miniature town, and full size train cars to explore. Sonoma has the plaza downtown with the last Mission, military barracks, the Cheese Store, the site of the Bear Flag revolt. It also has a Garden on the main road that is made up of unique and fun gardens for kids to discover. We really had fun on this trip.
If you go to monterey, be sure to go to Dennis the Menace Park. Just outside the park are paddle boats. Even My Museum would still be fun for 6 and a half year old. My 9 yr old did fine there. There is also fisherman's wharf--not much for a building for kid activities, but certainly neat to eat lunch and go to the candy store and watch taffy being pulled. Include one nick-nack store with nature glass wood animal stuff as entertainment. The Wharf scenery is also a novelty.
In Sacramento you can take a tour of the Capitol, walk old Town and pan for gems, watch the bridge move for boats, go to the train museum. You can even take the train to Sacramento.
Don't forget about riding bikes across Golden Gate Bridge, and the Children's Park in Golden Gate Park. Zeum (Children's tech museum) is good too. Taking a trolley, riding the curved escalator in the mall at Powell St. You could take a day trip to Muir Woods.
Spend the night in a bunk bed at Montera Lighthouse and spend the day at the tidepools or on the beach just below the lighthouse, that beach is really fun because of the stream going to the beach.
Take a hike at Russian Ridge, the wildflowers are out. Or bike ride at coyote creek (long paved bike path). Go to Berkeley to the Lawrence Hall of Sciences and the Botanic Garden. There is also a park on the marina that kids build with boards and tires.
There are tons of things to do and places to go that are close enough to drive to in a few hours. I could keep going, but I have to get out there today to have fun, too!