Also, the weather in San Diego can be 80°F (27°C) in December, if you get lucky. Not that anyone is recommending taking Interstate 5 but during the winter, you may run into tule fog. This a thick visually impenetrable fog that YOU DO NOT WANT TO GET STUCK IN! This is just one more reason to take the Pacific Coast Highway.
Everyone forgets the quite lovely alternative of US Hwy 101 
I think we mentioned CA Hwy 1 (what I believe you’re referring to as 101) earlier.
Here is a wiki cite on it.
CA Hwy 1
Just to clear up a potential misunderstanding, both 1 and 101 are highways in California. The 1 is beautiful; is the 101 also nice for that particular drive?
To complicate matters, there are certainly some unattractive parts of the 101 (and for that matter, the 1 as well). To further complicate matters, the 101 and 1 merge at certain points. I didn’t design it…
Do this. I can not recommend it strongly enough. Take 3 or 4 days and do this slowly, it is *well *worth the trip.
I have been to the grand canyon, Highway 1 is better.
One other thing to factor in if you’re considering taking the 1: Are you prone to motion sickness? It’s very twisty for long stretches. My wife can’t stand it unless she’s heavily dosed with Dramamine.
I can’t imagine visiting the Grand Canyon during December. You’re likely to run into snow and temperatures far below freezing. Plus, driving from California to Arizona, if it rains, can make a long and boring trip a very long trip sitting in stuck traffic because of some idiot who was doing 70 on wet roads and got into a wreck.
Well, the 101 between Santa Barbara and Gilroy will completely remind you of old California. It’s quite rural with lots and lots of golden (green during the winter) hills, oaks, and vineyards. Santa Maria is the big town with some unfortunately huge generic housing developments. 101 is certainly worth a look-see it it’s own right. Read some John Steinbeck before making the drive, it’s kind of east of Eden.
The 101 splits off from the 1 somewhere around Los Osos.
Just a quick reminder about the size of things in the US. California alone is over 2/3rds larger than Britain & Northern Ireland put together. Its population is “only” about 40% of Great Britains’ though.
Even fairly middling-sized states like Arizona or Nevada are each geographically larger than all of Britain.
However big you think it is, it’s bigger than that. Not that that’s something we’re overly proud of, just a fact about travel in the American West that almost never fails to surprise tourists from smaller, denser parts of the world.
And as others have said, December won’t be warm. Think March / April in London & you’ll have the right idea.
p.s. Welcome back. We’re glad to hear from you again.
Native Californian here. Driven up and down between SJ and SD many times over the last 35 years. There aren’t many scenic drives in the world that equal Highway 1 between Monterey and Morro Bay. Further south, the coast road still has some nice scenery, but you don’t get the same “edge of the continent” feel.
There are spectacular December days on the coast. In fact, the chance of a sunny day are at least as good as in Summer. Storms come in all the time, but that’s always the chance you take when you’re on the road.
I think driving PCH from north to south is the best because of easier access to the ocean-view pullouts.
If you have the time, here are some places worth visiting along Highway 1:
Monterey Bay Aquarium - It takes several hours to see the whole thing properly, so don’t bother if you’re in a rush.
Point Lobos State Reserve - A nature lover’s delight. Truly a beautiful place.
Big Sur - This is more of an area than a specific place. You can see some coast redwoods in Los Padres National Forest.
Hearst Castle - Former home of the newspaper publisher William Randolph Hearst. “Xanadu” in the movie Citizen Kane was based on it.
Yeah, you can almost not hear the opening strains to High Plains Drifter as you drive, that time of year.
…and Nevada, even not counting the 5,000+ square miles of radioactive wasteland and bombing ranges.
You can tour the radioactive wastelands, incidentally. Eh? Eh? See the remains of those houses from the nuclear effects tests and the atomic excavation crater? How’s THAT for “off the tourist path”? 
So that’s where Elvis went. A nuclear crater.
Hey, don’t forget Chico! A gem up in the North Valley, that’s us. Stop by and wade in the creek. 
I’ll take you out to lunch or something.
If I had meant California Hwy 1, I’d have said, Hwy 1. 
Especially since I used to live within a couple miles of it for about 10 years!
The reason I brought up 101 is that everyone was making it sound like the only alternative to 1 was the 5 up the Valley. So I offered the 101 as an alternative; while it isn’t the incredibly scenic (but much lengthier!) drive up the coastline from SLO to SF, it’s much prettier than 5 is (which has as its only redeeming quality the area between Newhall/Saugus and the Valley). It also follows the old Mission Trail, so you can stop off and see any number of the old Spanish missions along it, several of which are well-preserved still. And it’s not much slower than taking the 5. 
Ah, yes, Europeans tend to underestimate the size of things, and then think we’re crazy when we drive more than an hour or so.
When I was in Germany last year, we drove from Cologne to Tours, and then back. Everyone thought were were nuts, but it was nice, short, pleasant 7 hour drive including gas stops and excellent service center food (the way back was slower; we decided it would be stupid to be slow close to Paris and not at least see it).
In December of the prior year, we flew to Las Vegas, drove to San Diego, and back to Las Vegas. Again, a nice, pleasant drive. From Las Vegas to the the Grand Canyon would be a very short jaunt in relative terms. Heck, from Hermosillo we drove to Las Vegas and then to Lake Tahoe, and back, in addition to separate trips from Hermosillo to the Grand Canyon.
I’m not trying to diminish the size or scope of things, but merely offering a perspective that as an American, I was born and raised on the great American car trip. A lot of the fun is the journey in addition to the destination.
If you take 101, the stretch through the South Bay and Peninsula is the major commute corridor, and decidedly UN-scenic (50 miles of commercial/industrial zone). Bypass that on 85 / 280 (the much more scenic Junipero Serra Freeway). Living here, I take 280 in preference to 101 whenever it makes any sense at all, even at the expense of adding a couple miles.
That said, if you have the time, take highway 1.
I’d suggest taking 280 south out of SF (Well, start at the Great Highway and cut over to it). That way you can drive down the valley made by the San Andreas Fault. Since the water company owns a lot of the watershed, there are large stretches south of where 380 comes in that haven’t been covered by housing. As you pass Crystal Springs reservoir you can look to your right and know that the hills on the other side of the water are moving north while the ground under you is moving south, for a combined couple of centimeters per year.
Get off at Page Mill Rd and nip over to Stanford University for a walk around the Quad, and check out the Rodin sculpture garden. Then get a bit to eat in Palo Alto and head back to 280.
Time to choose, 1 or 101. (Note that I, as a northern Californian, do not put “the” in front of any highway number, but you probably know that already as it comes up in *absolutely every *thread about directions in this state :).)
A) If you want to continue south on 1, from 280 take 85 to 17 and go over the Santa Cruz Mountains to get on 1 in Santa Cruz. I echo suggestions about the Monterey Bay Aquarium, and also Carmel. The big fires this last season put a hurt on Big Sur, but there are so many miles of coast that the beauty has not been diminished.
B) If you want to continue south on 101, take 85 from 280 as well, just don’t get off at 17. Highway 85 eventually joins 101 in south San Jose. And then, as mentioned earlier, you start to get into Steinbeck country (well, you do in Monterey as well, of course). For a scenic detour, get off 101 just before King City and take Jolon Rd. It’ll go through the Hunter Ligget Military Reservation and past Mission San Antonio. Every Spring my grandparents would drive down to do Jolon Road, up until they were in their 90s, for the wildflowers. In December you will be much too early for that, of course, but the scenery is still worth it.
Jolon Rd rejoins 101 just north of San Miguel (another interesting mission). The next large town is Paso Robles, which in recent decades has become the center of yet another California wine growing region. Very good Zinfandels and Syrahs come out of there.
Both 1 and 101 eventually get to Santa Barbara. Maybe you’ll see some Gray whales in the Santa Barbara Channel at that time of year. When I was a freshman at UC Santa Barbara I’d sometimes take a chair out on the roof of the dorm and alternate studying and watching for waterspouts.
I also echo the “avoid greater LA traffic between 4 and 7:30” advice. It’s never any fun at any time of the day or night, but it is definitely horrible then.
In any event, one would have to hold a gun to my head to make me go down 5. Ninety-five percent of the time I will take 101, which although longer is much more restful.
If we are posting things to see on 1 you must stop off for. Give an hour or so to see the elephant seals.
http://www.amwest-travel.com/awt_pbseal.html
About 15 years ago they started coming onto the beach and now they are a huge colony. There is a board walk so you can get pretty close without disturbing them. December is a good time to visit as the bulls will be there throwing their weight around.
Very good ideas here already.
Don’t worry about the weather. It gets beastly hot here in summer, but we’re not prone to blizzards, ice storms, or hurricanes.