Are there any towns on the coast where you can park your car and shop/bar hop at outdoor restaurants? Similar to like a Laguna Beach or Hermosa Beach strand.
Is Belmont Park / Mission Bay worth some time?
You'll be in Carlsbad, and that's decent. Check out Pizza Port! San Clemente may also work for you if you are looking to walk around a beach/town area.
As I mentioned, Encinitas is good for that kind of thing. And if you are going to La Jolla be sure to check out the La Jolla Cove area for some nice beach.
Mission Bay is meh. The park is kinda ghetto, avoid it. I'm not familiar with Belmont Park though.
Just do Sea World when you are down there or if you want to check out a beach town in that area do
Point Loma.