The Model Railroad Museum in Balboa Park. If you will be in the San Diego area on a weekend, there is the San Diego Railroad Museum in the town of Campo an hour or so Southeast of San Diego. The Museum in Campo is run entirely by volunteers so it is only open on weekends as far as I know. A couple of hours North of San Diego on I-15 in Perris is the Inland Empire Railroad Museum. In Griffith Park in Los Angeles there is Travel Town with a bunch of smaller steam engines and trolleys on display. Pomona has a bunch of large locomotives on display in one corner of the Los Angeles County Fairgrounds. They include an ex-U.P. DD40x, a big boy and a challenger, an ex U.P. 2-12-4, and an ex Santa Fe 4-8-4, as well as other large steam engines. I think the DD40 is the only diesel there.
I don't know what other interests you may have, or if you have family going with you. San Diego's zoo is world famous. I don't know how well known the wild animal park is outside of California, but it is also interesting. It is owned and operated by the San Diego Zoological Society, same people who operate the San Diego Zoo. You can take a tram ride through a huge area where all sorts of African non carnivors are allowed to roam freely. There is also Sea World in San Diego, much more pricey than the Zoo. I think there is a decommissioned air craft carrier on display as a floating museum in the harbor in San Diego. Balboa Park is full of museums, arboretums, etc. No matter what your interests, you can probably find an appropriate museum in the park. The park is huge, but all of the museums are within a short walk of each other. You used to be able to buy a ticket book at a discount at the Museum of Science & Industry that gave you admission to all of the museums in Balboa Park. I forgot to mention that just North of San Diego in La Jolla (pronounced la hoya) is the Scripts Institute of Oceanography and Aquarium.