I was going to say visit Coronado, but mal beat me to it
The entire island is beautiful and there are a lot of fun shops and places to eat there, and the hotel is amazing.
I would strongly advise against Tijuana...I went as a teen with my family and I hated every minute of it. It was dirty, too full of people, and we didn't trust eating or drinking anything (we didn't have any bottled water, we should have brought some). People literally chased us out of their shops and down the street trying to get us to buy their stuff. It was a complete waste of time.
Also, San Juan Capistrano isn't too far away by train (Amtrak has regular trains in the area) if that is something you'd be interested in.
This post has made me really miss Southern California lol.