I live in San Diego County, and it is always fun going to the beach. I am sure you'd love Mission Beach, Sea World, also the harbor tours of San Diego Bay.
There is also Balboa Park, where you can visit museums, picnic, just hang out.
One of the most beautiful places to visit is Sunset Cliffs, which is right near Point Loma.
Another favorite place my daughter and I go to a lot, is the Shelter Island harbor to have a picnic and go for a walk.