Tomales Bay State Park - in particular, Heart's Desire Beach is a gem. Great beach for kids as there is sand bar so there is shallow area that warms the water for them to play.  Also hiking on the ridges and resident spotted owls to seek out.

The camping here I have used (and I think it is the only one) is a boat in site that is used by kayakers.  It is actually in the PRNS. It is not reservable thru Tomales Bay SP. We have come across Tomales bay from the opposite shore to spend the night.  There is also a land hike that gets you to the boat in site I have used to hike out and visit friends camping on the beach.  The beach camping is primative but there is a restroom.  Crossing the bay can be treacherous so study the weather - especially winds before you set out.  There are a couple companies who will guide you out as well.

Isolated camping but that is great as well.