Sounds like a fun trip.
San Francisco is a great city, with tons to see and do. it depends a lot upon what you like doing ... there's great people watching in The Castro (the gay district), and a great way to get there is using the street cars, which you can get from somewhere around Fisherman's Wharf. These are not the cablecars, which are also a must see and do.
As Tommy mentioned, there's the obvious places like Alcatraz, but there's also The Exploratorium. In 1992 I was with a lady friend; it was early spring and we were driving around The Castro. Her immediate observation upon looking at all of the male bodies was "what a waste".
Head up to Sausilito, and of course there's the Napa Valley, where a day or three in Santa Rosa will see you enjoying some fine Napa Valley vintages. Sacramento is California's State Capital and is an easy 90 minute or so drive from the Bay.
Access to Yosemite will present some challenges to you in January. It's winter, and much of it will be snowbound. Access will be through either Modesto (where we used to live) or further south in Fresno. Check the park's website. There will be some access, but much of it won't be easy.
Also, as you head into snow bound regions, in a rental car, safety items like chains may be required ... but difficult to come by in terms of what your expectations might be. My initial expectation (on my first trip to the US - 1982) was that I should be able to rent these from Mr Avis or Mr Budget, but I was wrong. I've not needed those items in that way since, but I think it's a point that's worth mentioning.
Highway 99 is the road that runs directly from Sacremento through Stockton, Modesto, Fresno and Bakersfield and into LA. Apart from the side trip into Yosemite, towns like Sonora, Jamestown and Coulterville, just between Modesto and Yosemite, might be worthy of a visit. Apart from that, there's not a lot in the Central Valley.
With all due respect to Tommy, the coastal road - PCH (Pacific Coast Highway), or Highway 1 - is spectacularly awesome, with lots of great and interesting spots to look at along the way. Carmel (some dude named Clint Eastwood used to handle the mayoral duties there a few years ago) and Monterey are great, Big Sur, San Louis Obispo (SLO), Hearst Castle (not my taste but possibly worth a visit on a girls' trip), and then there's Santa Barbara. Many of the villages have old missions - they form an essential part of California's history - and the missions in SLO and Santa Barbara are definitely worth a visit.
LA is a great city, but you need to learn and understand it; it can be intimidating.
I can't speak to what Tommy's experiences were, but I would suggest staying somewhere around the Santa Monica area. This will give you easy access to Santa Monica and Venice Beach. Muscle Beach is an absolute must on a girls' tour, and Venice Beach is ... different.
If it helps any, Jim Morrison from The Doors was from Venice. it's a very cosmopolitan, but still American, area. Try to figure that one out.
Heading back inland, get yourselves to Third and Fairfax in LA. That's where the Farmers Market is. Lots of great stuff to see and do. And eat. And spend $$$$ on. You're right in the La Brea area too, which means that there's the LA County Museum of Art, La Brea Tar Pits (yes, really) and the Petersen Auto Museum all within a short distance here.
For good down home tucker of the kosher variety, head to Kanter's, just a block or two up the road from the Farmer's market. We don't have these NY style kosher delis in Oz, and they're not just for the Jewish; they're truly an American institution and well worth a visit. It's winter in January, so grab the Matzo Ball soup.
Park La Brea is a residential area there too, bordering upon the Farmers Market. It's the largest rental/residential complex in the USA. Which also means in the world. High end apartments etc ... used to be clients of the people I worked with when we lived in Dallas, but I digress ....
In the hills there''s the Getty. Simply one of the best art museums to be found anywhere. Last time I was there there they had two photography exhibitions on, one being Weegee, and the second being "Photos For The Press". The power of the photo cannot be under-estimated: this was a small exhibit of maybe 40 or so images from around WWII on, and included images by Robert Capa from WWII. Most of the images were Pulitzer Prize winners, and many were images that we already know and recognise.
I don't know what exhibits will be on while you're there, but the Getty is one of my favourite art museums in the world, and I cannot recommend it highly enough.
Shopping ... depends upon what you're wanting, but Santa Monica, Farmers Market/La Brea, Beverley Hills and Rodeo Drive, and/or South Coast Plaza (OC) should see all y'all done.
Just south of LA you have OC; Dana Point and Laguna Beach might be worth a lunch or dinner stopover. A sunset dinner at Las Brisas is always a worthwhile experience, for instance.
San Diego is great too. Balboa Park is the home to many great museums (as well as the Zoo), but there's lots more in SD as well. Do visit Old Town, and the gaslight district. The Old Town Mexican Cafe has great food and is very inexpensive. The USS Midway is anchored on the harbour as a naval museum, and the maritime museum is a little further north. Mission Beach is fun too, and of course, Tijuana is, oddly enough, just south of the border.
Note carefully that I've also mentioned all of the cultural attractions I consider visiting in and around Las Vegas. if you've not been there, then by all means go, on the basis that it's a place that you need to see, so that you can say that you've seen it. Hoover Dam is nearby, and there's a new road that bypasses the Hoover that's just opened as well.
The Grand Canyon is near-ish by, and is spectacular. If you're in Vegas, do a fly-by, or else head towards Williams in AZ, and soak up some of the old Route 66, but that is slightly out of the way from the main thrust of where you'll be heading.
But otherwise, for me, Vegas is crass, ugly, and totally unworthy of anything relating to life, the universe, and everything.