Keep the weather in mind when planning a trip for January in California. The weather varies quite a bit around the state depending on geography and elevation. Coastal Southern California is well reputed for its mild winters. Daytime highs generally reach into the upper 60s and occasionally into the low 70s, and rain is rare. The southern deserts can be a tad warmer. The central and northern coasts will be cooler, and rain/clouds much, much, more frequent. The high mountains will be blocked with snow. Several mountain pass highways will be closed for the season, and other may be subject to chain restrictions at any time.

A few specific thoughts:

1) Driving the Pacific Coast Highway can be beautiful any time of year. Allow yourself at least a full day for it, if not 2. In the winter it helps if you can be flexible... the drive is much less fun on days with heavy rain. Plan 1/2 day to see Hearst Castle.

2) I don't see any reason to fly via SMF unless you have family or business in Sacramento. I suggest your northern terminus be SJC (closest WN airport to Monterey and the central coast) or SFO (to visit the amazing city of SF itself plus any of the nearby attractions).

3) Parks-wise, Yosemite Valley will be open, but it'll be cold and snowy. It's beautiful in the winter; just don't expect to be able to do a lot of hiking. Death Valley will be open, and the weather will be mild. If you haven't been before, the winter is a great time to go! Also consider Joshua Tree National Park and some of the Indian lands around Palm Springs.