Winter isn't the best time for Yosemite since the best hiking in the high country and at the Glacier Point area will be off limits most likely due to the roads being snowed over. However, the Yosemite Valley is still an awesome place. A basic winter visit could be 2-3 nights, with a hike up to Nevada Fall on one of the days. Beautiful area and not too bad a ball-buster, being about 7 miles roundtrip with maybe 2000 feet of elevation gain. If the road to Mariposa Grove is open you might consider making it 3 nights, as the hike through Mariposa Grove to see the Giant Sequoias there is pretty amazing, and a very easy 7 mile roundtrip if the snow isn't too deep. Then again, you guys are in shape so if you have ever cross country skied, you could do a cross country ski trip from from Badger Pass to Glacier Point, spend the night at the
Glacier Point Ski Hut, and then ski back to the Badger Pass ski area. I'd aim for no earlier than mid December, as in a low snow year the ski area might not be open until then (though in a historically low snow year like this one was for California, it might not open until mid January). The view at Glacier Point really is really incredible; I think it's the best view in the lower 48 of the US that can be driven to, including the Grand Canyon. Plus they have groomed ski tracks to it in the winter after the ski resort opens. You can also snow-shoe, but I find snow-shoeing to be very slow I don't know about making that distance in short winter days. Also get tire chains, which you will not be likely to need but will be required to carry to drive into the park. If you're comfortable with winter camping in snow I can offer some more suggestions.
But if the Giant Sequoias are really important to see, Sequoia National Park and Kings Canyon National Park all over Mariposa Grove in Yosemite. The Giant Forest is unreal to drive through, as the Giant Sequoias are everywhere. Not sure about which roads are open, though I think the main one is open year-round.
In San Francisco make sure to drive up to Muir Woods, not too far across the Golden Gate. They have Coastal Redwoods which are cousins to the Giant Sequoias; the Coastal Redwoods are the tallest trees in the world, while the Giant Sequoias are the most massive. Dungenness Crab and cioppino are not to be missed in San Francisco if you love seafood. Unfortunately my favorite restaurant in SF closed a few years ago, so I don't have any great recommendations.
For San Diego, I thought the world famous zoo was crap. If you're into aviation they have the best air and space museum I have ever seen; even better than the national one at the Smithsonian. And you could visit the aircraft carrier and see why David Robinson got that exemption to not have to serve 5 years after graduating the Naval Academy.
For the LA area my favorite thing to do is to hike Mount San Jacinto near Palm Springs, about 100 miles East of LA. The peak is 10,800 feet or so while the Coac a Valley is only 500 feet elevation, so the views from the summit are airplane-like if you hike it on a clear day. If you do it before mid December you have a reasonable shot of it being snow-free. Big Bear Lake probably has the best skiiing in the LA area. Joshua Tree (also near Palm Springs) has great rock climbing, whether you're talking technical with belays or easy class 2/3 scrambling.
The two best views of downtown LA are at the top of the short hike from the Griffith Observatory and from the parking lot at Dodger Stadium. That view near Griffith Observatory is awesome because you're on the ridge separating the valley from Los Angeles, and in each direction you see huge cities.
I have never driven the Angeles Crest Highway in the winter, but if it's open, it's the best drive in the LA area. Mount Wilson has a great view down onto LA on a REALLY clear day, but those are very few and far between in my experience (no hike required; you can drive to it). Since the water will be way too cold to swim or surf in, the best beach is Zuma Beach near Malibu. It's got a nice view from on top of a rock that is an easy walk up. If you were going in summer I'd recommend Huntington Beach for its large waves, but you probably get better in Australia anyways.
Santa Barbara isn't my cup of tea, but the GF will probably love it if she's into wine.
For the best skiing you might take highway 395 up from LA to Mammoth. It's just outside Yosemite, but the road into the park will be closed and there is no getting across the Sierra Nevada in the winter by car; you have to drive around it. Or you could catch it coming in from Las Vegas. Make sure to take a drive through Death Valley on the way if so.
Don't know about Tahoe. Not a fan of Las Vegas. Spend no more than 2 nights in San Antonio. It's a cool place to live since it has dirt cheap cost of living, no state tax, lots of Mexican es and Mexican food, but no way does it deserve taking time away from areas like San Francisco, LA, Yosemite, San Diego, New Orleans, etc. Better place to live than to visit definitely.