I almost suggested Hearst Castle, but I thought your plate was already pretty full. If you go, make reservations as far ahead as you can -- they fill up. Also, since you have a little one, take note that some of the tours have a lot of stairs to climb. Tour 1, the intro, is probably best. You'll burn at least 2 1/2 hours by the time you park, wait for the tour, etc. But it's a sight you'll see nowhere else.
Wow - great info here. DH and I will be doing same drive, but without the little ones (San Fran to Disneyland). We are debating the same thing - staying overnight in Monterey vs. farther south. We will be leaving from SF early am on a Saturday, checking in to Grand Californian Sunday evening.