Catch the Blue Line Trolley anywhere downtown headed South takes you right to the border. Cross into Mexico on foot and walk into Tijuana about 15 minutes with tons to see on the way.
Bring passport
Stop into where they invented the Caesar Salad.
About $2.50 each way last time I was there.
Get ready to be swarmed by kids selling Chicklets 2 per pack and donkeys spray painted to look like hot pink Zebras.
Something perverse about what goes on down there, but hop on the rail and you are back to your hotel in 1/2 hour
There is a fantastic restaurant in San Diego that is built slightly below the water line. When it gets dark the restaurant shine lights on the water and the baitfish swarm everywhere with birds going for the easy pickins.
The name will come to mind.
Old Town is also ok to spend a few hours
Too bad you were not there on April 19th. It was the Avocado Festival in Fallbrook. Crazy times. I try to be down for the festival every 5 years or so.