Here is a list of my favorite places to visit:
Laguna Art Museum
This museum has a very contemporary collection. You won't find too many portraits or classical paintings, but if you are in the neighborhood I would stop bye to see what's new.
Laguna Beach MuseumIt's worth a drive to tour the museum and then take a nice stroll along the beach. The museum has a nice permanent collection of works by California artists and brings interesting works from contemporary artists.
Orange County Museum of Art
Usually has very contemporary exhibits.
The Bowers Museum of Cultural Art
California's Golden Years period (1915 to 1935) highlight the museum's spectacular collection mostly of plein air paintings, but there are some nice portraits as well.
The Irvine Museum
In Los Angeles
Los Angels County Museum of Art
J.Paul Getty Museum
San Diego Museum of Art
Has "The Shepherdess" by William Adolphe Bouguereau!