Another place we would never have thought to go, if it wasn't for the recommendation from friends, was San Juan Capistrano. It's an old Catholic mission station that is one of a number that stretched along California from when the Spanish came to catalogue and convert the land of Mexico - as it was then. Now it is open to the public and an informative guide gave us the history of the place. Lovely grounds, interesting chapel, and slightly odd gift shop. I think there was talk of it being the oldest building still in use in California. Yes, just checked, and if Wikipedia is to be believed, it was built in 1782. By Californian standards, that's as old as it gets.
A baking hot day, we called in on the way to San Diego, having a great lunch before we carried on to our destination.