Have you signed up for BAHFT yet? That's a yahoo group that publicizes this kind of stuff for the whole Bay Area. Also, frequently it's a means of getting tickets to cooperative field trips that are excellent--some homeschool parent will sign up for a 'classroom' type spot and then put the tickets out on BAHFT for others to purchase. It makes up a critical mass of kids, and gets you exposed to excellent science and arts events. I particularly enjoyed these field trips to the San Francisco Ballet, San Francisco Opera, and the Lawrence Hall of Sciences in Berkeley.
Let me also suggest that you join the Exploratorium in San Francisco--it's an absolutely outstanding science museum for older kids, with the best biology and physics interactive displays I have ever seen anyway. The Children's Discovery Museum in San Jose is fine for younger kids, but the Exploratorium is far better for older ones.
Also, sign up for the mailings of the MidPeninsula Open Space, and you'll be notified of local nature hikes.
Now is the time to plan your summer camps. You might consider the Seymour Center in Santa Cruz for excellent marine biology camps. Fairyland in Oakland has outstanding drama camps.