Favorite off the beaten path beach in California?

Not sure how far north you want suggestions for....but I simply LOVE San Gregorio Beach. See my review about why.

I second San Gregorio. I also love Pescadero St. Beach and Bean Hollow. The best for little kids is Pigeon Point. It is a quiet and sheltered cove with very small waves. The Pigeon Point lighthouse is fun to see as well.

Another great beach that not as many tourists know about is Muir Beach. Funny considering Muir Woods is just down the street and a huge tourist draw. Point Reyes is also full of great beaches that nobody is ever on. Since these are Northern California beaches, they are definitely not the swimming type. One other fun Norcal beach is the one across from the lighthouse in Crescent City. Its great for people who love to go beachcombing and at low tide, you can walk across to the lighthouse!

My favorite is Ribera Road Beach, off Hwy 1 just south of the touristy part of Carmel.
It's a public beach tucked away in a residential neighborhood, and is not usually crowded. It has nice sand, but there are a lot of steps to get down to it, if you're toting a lot of stuff.

While it is a *bit* touristy, I love going to the state beach at San Clemente. I grew up camping at this beach and absolutely love it for it's beauty both down at the water, and up on the cliffs where you camp.
It is a very long and a bit frightful walk down the hill to the beach for young children, but older kids usually can do it just fine. There are lots of hiking trails in and around the mountains and bluffs, and although there are quite a few families on the beach in the middle of the day, the evenings can be a beautiful, quiet and almost solitary experience.