Unless you live in Las Vegas, all night diners are tough to come by. Sometimes it's nice to get out, or stay out, and then go have a burger, or dessert, or whatever, but if it's after 9:00 p.m. in the 'burbs, that is NOT going to happen.
Harbor House in Dana Point is the coolest place to hang out, especially late at night. The beachy, low-pro casual atmosphere is so totally comfy and pressure-free. The food is great, and portions are HUGE, but they're happy to give you a container to take the leftovers home for a rerun snack the next day.
The inside of Harbor House is really cool - old time leatherette booths, and movie star pics all over the walls and ceiling. And the graffiti is to die for! (Okay, that was weird and just jumped out).
But mostly, it's just a cool, fun place to hang out near the beach...especially in the middle of the night when the rest of the world is sleeping.