If you have a really long layover (4 hours), there are direct buses that go to Santa Monica and 3rd Street Promenade - the Big Blue Bus is what it's called - Traveler's Aid downstairs by baggage claim can tell you which to take (they are also a source of other stuff to do near the airport). You could do it shorter, but you'd want time to hang out once you got there.

Cheers,
Irene