Howarth Park in Santa Rosa--we may be visiting here when we are in the North Bay--most of the cool extras are opening in February, but it's still an awesome park. Maybe next year we can do part of our visit in February.
Palo Alto Junior Museum and Zoo--very small and inexpensive (free, with $3 suggested donation), with a nice playground and the local Children's library right in the same area! Nice for a mellow day close to home (or Grandma's house, in our case).
Oakland Zoo--we really enjoyed this!! Lots of great animals with plenty of space, a fun kids' play area "Children's Zoo", and an adjacent rides area with carnival like rides--my favorite was the train ride overlooking the zoo and going through an area with wallaroos and emus, the kids also liked the carousel and airplane rides