Ad I've said before... my dream kitchen. One of two at Filoli...
Second in my thoughts is Ardenwood Historic Farm. I've been on dozens of picnics here since childhood and even picked tomatoes in the summer with my Mom and Grandma in the fields outside the park. Throughout the year the farm hosts period appropriate activities like making ice cream in a wooden barrel, gardening, and going on hay rides among other things. The Victorian house on the property can be toured and there is often a beekeeper selling local honey. On my last trip I met their resident "wild" fox who has been hanging out for a few years. He was so close I could have touched him, but took this pic instead!
A place I recently discovered and loved, was the John Muir House in Martinez. The house is beautiful, and you can walk up through the attic all the way to a glass enclosed Widow's Walk which would be ideal in a rain storm with a cup of tea. The orchards beyond are spotted with persimmon, apple, quince and fig trees. At the back of the property is an Adobe house which is also a museum on local history. Its a true treasure!
If you are in Martinez you might as well continue onto Walnut Creek and visit Shadelands. The House here has a unique history and is quite beautiful. I discovered it because it hosts a biannual antique show which is one of the best locally. If you can swing a trip when the antique show is on, you are in for a treat!
Another goodie is the McConaghy House in Hayward. This is a place I haven't been to in years, but remember in fondly from childhood field trips and visit during Christmas when it was decked in greens and ribbons. There are so many treasures here in the Bay. You're only a google search, a car ride and a picnic basket away!