This would seem to me to be the age-old problem of taking a short trip to a place that requires much more time to see. Do you treat the trip as a scouting trip or do you spend quality time in just one place. Since you have already been to Califorina, you may have already scouted the place and know where you want to return to, but weather may not allow that. When presented with this problem, I've always tended to make it up as I went along. One thing I like about the western states, is that the places YOU find most interesting just seem to pop up almost at random as you travel around. For me, some of these places aren't even in the guidebooks and I would never have found them unless I just happened upon them.
If you end up doing the coast route, LA gets a bad rap but in my opinion has some nice places to visit. If you drive through there and have time you might drop by The Getty and the Huntington Library.
Denis - good points there. I like to take the back roads and be surprised, leaving enough time in the travel schedule to stop and look around. In a large city I prefer to have company but on the road with a camera I'm quite satisfied on my own.
Thanks for the ideas and links so far. At this stage I'm leaning towards heading back to the coast and spending a few consecutive nights in the Carmel area.