Are you guys actually NOT going to any theme parks? I don't think that's bad or good, it just would be an interesting diversion from the typical first time L.A. traveler is all. If you guys want some cool hiking/exploration spots and a chance to escape the crowds that you'll see in the beaches nearer to where you are staying I suggest:
Probably my favorite place in the world. Just some downright awesome trails at those places. A lot of offbeat stuff to find. Cool caves, water holes, horse trails, best surf spots , whatever you want to do in the outdoors. It's good stuff.