Search over at the largeformat group. Someone over there used to live in Oceano. Oceano is a tough spot to get good weather for dunes for a week. If it is foggy, then no shadows. If it is clear, then often still and the number of foot prints will be great. I think you can still ride horse in the 'no drive' preserved area. It is not a very large area. When they are right, they are wonderfully sculptured sand.
There is lots of camping along the coast, if that is what you need.
If you just want dunes, then Death Valley has 5 sets and the Devil's Playground (south of Baker at Kelso, Calif) is probably the largest patch of sand dunes in North America.