I actually just did that drive again 1.5 weeks ago - 3 days with 2 overnights is plenty, we did it with just one night at a hotel. On the way up we stayed in Monterey and on the way back to LA we stayed in San Luis Obispo.
On the way up, we stopped by Santa Barbara (beautiful town!) to eat, walk around, and so I could catch the EuroCup finals. We also hit up Paso Robles on the way up, which is nice if you're not going to hit up Napa...it's got a very different feel to it, but the wine is still great. After staying in Monterey one night, we did the 17-mile scenic tour around the city, along with dropping by Carmel. Both are beautiful cities with Carmel being a little more quaint and high-end.
On the way down we hit up Heart Castle and that was amazing. Flat out amazing. Other than that, I can't say San Luis Obispo was good or bad as we got in rather late and only went out to dinner. Slowly made our way back to LA the next day.