I don't think the beaches in LA actually close, but there's no loitering between 12am-6am, which I have no idea what that means. Never really been to a beach after 10pm.
I think the location of an afterparty will have to depend on how many people go, who's going, what people want to do, etc. I'm not really that familiar with the metro LA nightlife scene. But I do know of a karaoke place in little tokyo with large rooms. There's also lots of places to eat, everywhere.
As for shopping, there's 3rd Street in Santa Monica, Beverly Center, and Citywalk.
This is all in LA btw, there's still a whole lot of crap in the OC for people willing to travel for a little bit. But I don't think it's really worth it unless you're going to Disneyland or something.
Like I said in the other thread, Yelp.com is a good resource for finding places. Can see ratings and and filter by location and price.