As noted already, a very large area. What are your interests? Scenery, history, food?
Plenty of cheaper but clean motels, don't rent, unless you want a mountain or redwoods or beach retreat for a while.

I have visited all of the Missions, Spanish history being an interest.

I enjoyed visiting the gold rush towns. Up the coast,the area around Eureka, an old town of the lumber barons, and where the Russians had a trading post. Sacramento has a fantastic railroad museum. I also enjoyed wandering the floodplain of the river, looking at plants, etc.(not everyone's cup of java). The tremendous expanse of the irrigated rice fields near Sacramento was amazing to me.

I enjoyed the train excursion through the Napa Valley, and visits by car to some of the wineries (Beringer's is very commercial but visit the old storage caves, in a part of one are preserved racks of bottles that survived the widespread effects of the SF earthquake of 1906, too dangerous to retrieve. The town of Napa itself is ugly).

I always enjoyed driving from the coast over the high country and into the Central Valley- several routes to take but not the heavily traveled ones.

In San Francisco itself, the old mission on Dolores, SF de Asis, is well-worth a visit. San Francisco would take a couple of weeks in itself to fully appreciate all it has to offer.

Get the AAA "Northern California & Nevada Tour Book," well worth looking through while planning a trip.