San Diego Automotive Museum
It seemed like most of the free places yesterday in Balboa Park were only free for San Diego residents, not for other people. They only check the San Diegan in your group, if you all claim to be from the same place.
Anyhow, the two places I found of interest yesterday were the Automotive Museum and the Air & Space Museum. The Automotive museum had a 1947 Cadillac that was designed to drive from San Diego to New York round-trip without stopping. It has the capability to refuel while moving as well as pump its tires while moving.
At the Air and Space Museum, I enjoyed the exhibit about the Flying Tigers and the effort to fly supplies over the hump into China while Japan blocked aid from coming in via land and sea. They also had a German Tri-plane which, while allowing planes to manuver more tightly, tended to lose the top wing.