During my in-laws' visit, we spent a day in Pasadena (the home to the Rose Bowl Stadium - host of the 1994 World Cup and the biggest monthly flea market) to finally visit The Huntington Library.
We knew that we would see beautiful gardens and a great library collection. And that's exactly what we're given: nice walks through plants and flowers and, in the library, some of oldest manuscripts (and other materials) of our history!
Go in the morning and then visit Old Town for lunch. There is a cool street with nice shops and restaurants. Definitely a good day!