im not sure about the closest cause you aint exactly on the coast but id say go north on 101 to 92 west to half moon bay. there are hella secluded beaches right there in between hmb and pacifica off devils slide.
it will probably take a half hour or so to get there but from where you are, you probably couldnt do too much better. i dont think you could really have a party if thats what you want, but just smokin with a couple of friends would be great.