There are two state campgrounds right on the beach, and both of them are really nice:
San Elijo State Beach
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www.parks.ca.gov
2050 N Highway 101, Encinitas - (760) 753-5091
South Carlsbad State Beach
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www.parks.ca.gov
7201 Carlsbad Blvd, Carlsbad - (760) 438-3143
San Onofre State Beach, North of Oceanside
949-492-4872
And a quick google search yielded these names for beach camping:
* Guajome County Park
* Oceanside RV Park
* Paradise-By-The-Sea RV Park
* Trailer Rancho RV Park
If you've never seen the 200" Hale telescope on Mt. Palomar, there is a state park there as well, and the observatory is awesome!
Roger