California is a state in the United States of America. Hitchhiking here is easier than in a lot of other states of the US. You usually won't be harassed by cops.
Vallejo seems pretty ghetto, though the rest stop in Vallejo is really good.
Humbolt county is real good. They have a Hoopa Indian reservation up there that is pretty good. They'll stop if you're walking on the side because they think you're one of them, and then once they stop they feel bad and give you a ride anyway. Lord Ellis peak took me about half an hour to get a ride to Redding.
The following places are recommended: Big Sur, Jade Beach, Crescent City has a beautiful shore/beach. Yosemite, Sequia National Park, and the redwoods are a must. Haight and Ashbury in San Francisco is long gone and beat like a drum, but you might meet a few interesting people there. Arcata is mad cool as well.
Also, if you are heading down (or up) the coast, try and get to Mendocino. It's the county in Cali that produces the most marijuana, and there is this awesome communal farm there.
Eureka is a coastal town in California, on Route 101.
There's a gas station and big truck station, close to a nice traffic light. The people working at the gas station and truck station seem pretty nice, and don't hesitate to ask around a bit. Your best bet, however, is to simply wait at the traffic light with a clear sign. You're likely to find a ride straight to the Bay Area.