Between San Diego and Orange County, you can camp at San Onofre State Beach. You'll need a reservation, which go quickly. San Onofre State Beach Webpage.
It is right next to a nuclear reactor, so that might put some people off.
Pro tip

Between San Diego and Orange County, you can camp at San Onofre State Beach. You'll need a reservation, which go quickly. San Onofre State Beach Webpage.
It is right next to a nuclear reactor, so that might put some people off.