Salton Sea is not technically LA, but is a by-product of its history and my first-choice escape from the city: one of the world's largest inland seas and lowest spots on earth, a man-made lake in the midst of the Colorado desert, a resort area in the twenties turned in one of the worst ecological disaster of the past century.
Malibu's Point Dume Beach has got to be my favorite all time beach. Its a great place to get away from the city and leave your troubles behind. The beach mostly is a private, but you will definitely find beautiful seals and surfers.