Driving the coastal route south can be difficult because it changes names so many time.
From Long Beach you want to head out Ocean Blvd to 2nd Street to PCH and then head south.
Maybe stop at the Huntington Beach Pier, Laguna Beach Main Beach area, and stay along the coast through San Clemente until you HAVE to jump on the 5 Freeway to cross through USMC Camp Pendleton. Get off the 5 at the very first exit in Oceanside and then hug the coast the whole way down.
People come to So Cal and spend an entire week in cities like Huntington Beach, Newport Beach, Laguna Beach, Carlsbad, Encinitas, Solana Beach, Del Mar & La Jolla so it is very, very hard to say what you can stop and see on a drive by.....
Jake's Del Mar would be a nice place for lunch. You are literally on the beach.