Here is a list of day trips/activities one can do with one’s kids (kids tested in the 3-8 year old range) , as long as one lives on the Peninsula, and one is willing to get to those places fairly early (10 AM ish).
Don’t forget to pack up a coat and sun screen.
SAN FRANCISCO:
San Francisco Zoo (be sure to arrive at 10AM when they open, and bring coats/hats. It can be really cold).
Fisherman’s Wharf (park at Ghirardelli Square, walk to the wharf)
Ghirardelli Square (Be sure to visit the ice cream place on your way back from the Wharf)
Aquarium of the Bay (Tourist trap, but just the right size for the kids)
SANTA CRUZ AREA:
Roaring camp on the way to Santa Cruz (for the train aficionados).
Be sure to go on the Wharf all the way to the end to look at the sea lions.
Another interesting, but in the tourist trap category in Santa Cruz is the Mystery Spot.
Henry Cowell Redwoods State Park (as good as Muir Woods, but less crowded/far)
SAN JOSE:
Bring at least one extra set of clothes. The water play area is great, but wet (Duh!)
GILROY:
Gilroy Gardens
Is a weird combo of horticulture theme park. The rides are just right for younger kids, and the weather should be just right this time of the year.
BELMONT (this weekend only)
SAN MATEO
Coyote Point Park
Has a great play structure called Magic Mountain, and you can fly kites on the beach