Been to the zoo, different from the Singapore Zoo, but it is still a zoo. I found it pretty boring.
Balboa Park is great but parking is a pain. La Jolla great bars, restaurants and shopping. Great place to wander and waste time.
Tijuana is pretty cool, cheap booze, cheap ...well everything. But just be careful or your surroundings.
Sea World is definitely worth going, if you do have more time an hour drive up north might be worth doing to check out Anahiem, Disney World and the other theme parks out there.