As a resident of San Diego I highly recommend spending at least 2 days in Balboa Park. In addition to the zoo, it’s home to a dozen museums and numerous gardens. It’s the largest urban cultural park in the country. The facades on the buildings were used in the movie Citizen Kane to represent Kane’s fictional palatial estate of Xanadu. It’s worth a walk along The Prado, the park’s main walkway, just to experience the architecture.
Another hidden gem in the park is the San Diego Model Railroad Museum, one of the largest model train displays in the world. It’s 32,000 sq. ft. of world class models in a variety of scales many of which were constructed from the actual blueprints of local San Diego buildings. The Tehachapi Loop exhibit is two stories high and models almost turn by turn the train line from Mojave to Bakersfield. Quite a sight.