If you're planning on doing much driving in higher elevations, make sure you rent a 4-wheel-drive vehicle with snow tires. You won't need it if you're planning on staying near the coast, but if you're thinking about a day trip to, say, Lake Tahoe, you'll need it.

The higher passes, such as Donner Pass, are patrolled during snowy conditions and you can be told to turn around if your car doesn't have snow chains or 4WD with approved tires. Rental cars are not only not equipped with snow chains, their use is prohibitted by most rental contracts.