I'm going to second the endorsement of the Central Valley refuges. They're right up there with Bosque Del Apache for the variety and numbers of birds. Want to see a million geese in one place? Go to Klamath. Klamath and Tule Lake also host the largest collection of bald eagles in the lower 48. The major refuges from south to north are San Luis/Merced, Colusa, Sacramento, and the Klamath Basin, but there are at least a dozen smaller ones worth checking if you have time time. More sandhill cranes at Merced than the other sites, but thousands at least at all the major ones.
In the south I hear the Salton Sea area is good, though I've never been, ebird.org is a great resource to see what's been see where lately.
The coastal areas have a great collection of seabirds, and depending on the season and how strong your stomach is the pelagic birding boats are great. In season places like the Farallones are mind-boggling. Trips from Monterey/Moss Landing are supposed to be good as well.
Point Reyes has a great combination of land habitats with ocean, everything from tule elk to elephant seals and humpback whales.
Don't tell anyone, but the best part is that during the winter you're likely to have minimal crowds, if any. I've shot cranes taking off at dawn at Merced and been the only one there.