A little more information:
From Portland, Oregon to Eureka is 400 miles. Because 199 is narrow and windy, I say it take 7 to 8 hours to drive, with minimal stops. But you may want to stop (talking California only, you can sight see in Oregon another trip) at Jeremiah State Park for a picnic in the redwoods. South of Crescent City is a little tourist trap called Trees of Mystery. It has a giant Paul Bunyan and Babe, the Blue Ox. In the right mood, with a group of people, this place can be a giggle. Just before Orick is Lady Bird Grove, a wonderful redwood grove with magnificant ferns. You can see the largest herd of Roosevelt elk near Prairie Creek (late afternoon is good). 35 miles north of Eureka is the seaside town of Trinidad. In addition to some great beach access, there is now an Indian Casino in Trinidad that has a great resturant–very good food and a great view. In Eureka, you may want to spend some time in Old Town–I’m sure the link Zenster provided will give you details (best store is Humboldt’s Finest, selling items from regional artist of all types, really beautiful stuff!). There’s a fairly good brewery in town–Lost Coast Brewery–try the Alleycat Amber. You could drive to Eureka and see the sights (including Ferndale and the Avenue of the Giants) in a long day and a half or two days, but if you wasnt to take a long walk in the redwoods or on the beach, plan on two and a half days.
Between Eureka and San Francisco is about 350 miles, on 101, we always say it’s a 6 hour drive (if you don’t hit traffic south of Santa Rosa); going on Highway 1 will add some miles, and time–it’s a narrow road and the view is fantastic, so you won’t want to drive fast. On 101, this leg shouldn’t take more than a full day, and that includes cutting over to Napa to spend the night–plan a whole day for Napa Valley, it’s pretty and several wineries are worth taking a bit of time at–a few have deli food, so you could save some money and plan of a picnic lunch. If you go to Mendocino, I say that’s a day from Eureka (with time to look around), then half a day to get down to Napa.
Oh, this brings us to weather. The Northern California coast is foggy most of the year. It will burn off in mid-day (sometimes), but you need to be around in August and September to even think about depending on sunshine. Napa will be much sunnier much earlier in the year–by the end of April you can usually count on good weather most days. South of San Francisco, still foggy until you hit, well, say Santa Barbara. Again, when 101 heads inland, you’ll have nice weather most of the year.
Have fun, and spend alot of money in Eureka–my family lives there, and anything to help the economy is greatly appreciated!