Though one of the smallest museums in the entire Balboa Park district, the Tinken Museum has an edge on some of the other, larger places -- it's free. All the time, every day.
In addition to special traveling exhibits (such as the Byzantine Icons exhibit of varied religious altar pieces and icons when I was there), they play host to the Putnam exhibit, which includes artists like Rembrandt and Van derKlee.
It's open from 10 am until 4:30 pm on the weekdays, and 1:30 - 4:30 on the weekends, closed on Mondays and all of September. Donations are requested, but not required, and they do sell catalogs of the current showings for a reasonable amount (under $20, full of inspiration).
As I mentioned, it -is- a very small museum. But for a child or an adult just wanting a little shot of art to fill his or her afternoon, the Tinken is one of the best places to go.