First, do not confuse Sonoma with Sonora. They're different places. They're both good to visit, but they're a few hundred miles apart. :-)
I've already given the following advice:
Welcome to our beautiful state! I should warn you, it's pretty big. If you're doing San Francisco, Sequoia, Yellowstone, and Sonoma, you're going to be spending a lot of time in a car. Nothing wrong with that--we Californians do it all the time--but rent a comfy one.That being said, here's a list of stuff that's just off the top of my head.
First, Northern California's most famous historical event was probably the Gold Rush. In downtown Sacramento you will find, among other things, Sutter's Fort--the place where the first nugget of gold was discovered in 1848. The white people who were already here got filthy stinking rich. But by the time news trickled back to the East Coast and kicked off the Gold Rush in '49, most of the best claims were already staked out. The ones who got rich at that point weren't the gold-hunters, but the people who mined the gold-hunters: the merchants and tradesmen who made and sold the clothes, the tools, the booze....
While you're in Sacramento, there are plenty of other things to do. One that my family has enjoyed is the Railroad Museum. It has many locomotives on exhibit, and many different kinds of railroad cars--including some very lavishly appointed dinner cars. Next to this, airplanes just don't stack up as a very civilized way to travel. And apparently there is a 6-mile steam train excursion that follows the levees along the Sacramento River, open from April through September.
Aw, heck: here's the website for the Sacramento Museum Guide. You can find stuff to do in Sacramento for yourself.
Of course, the Gold Country is much more than just Sacramento. There is a one-time boom town, that was one vote away from becoming capital of California, before it went bust. That town is Columbia. The town has been maintained in its 19th-century state so you can see what it looked like during the Gold Rush.
Of course, before us Gringos came, there were the Spanish. They didn't settle the land in the same way that the Anglos did; they primarily left small military outposts surrounded by large rancheros and lots of open land. Aside from that, civilization in the state centered around the Catholic Missions for quite some time. There were twenty-one of these, mainly located on the "King's Highway" (El Camino Real) which ran from what is now the Mexican border all the way up to the border between Spanish and Russian California. Incidentally, the northernmost of these is Mission San Francisco de Solano, located in the town of Sonoma. Several of these twenty-one missions are still in use as churches; most of them are still around as state parks and are open to the public, although some are in poor condition.
Speaking of Russian California--yup, it's true. For a while Northern California was lightly colonized by the Russians. The Russian colonists even had some contact with John Sutter (the guy who founded Sutter's Fort); but they were gone by the time the Gold Rush started. They were mainly trappers and the like, who didn't put down roots. But they did leave some important historical sites--most notably, Fort Ross, which is also in Sonoma County.
If you get tired of historical sites and want to check out geologic sites, California has a whole lot more geology than is really safe. We've got some active volcanoes, including Mount Lassen, which last erupted in 1915. It's absolutely beautiful country; but if you go, beware that most of the really cool stuff (fumaroles, mud pots, that sort of thing) will still be under snow in late April. (They'll still be under snow in late June. Tonya and I know this first hand; we visited some of these sites on our honeymoon.)
There's also Mount Shasta, which is one of the most beautiful cinder-cones you could ever hope to see. Mt. Shasta City is a beautiful little spot about three hours north of Sacramento. Be warned: the town is a veritable Mecca for the sorts of people who like to use crystals to channel the lines of energy that converge harmonically on the Sacred Mountain. If you meet them, smile at them and think peaceful thoughts, m'kay?
A little north of Sonoma, in the Napa County town of Calistoga, there are a few interesting geologic features. For one, we have our very own Old Faithful Geyser here in California! It's not as famous as the one in Yellowstone, of course, but ours actually goes off a bit more regularly and a bit more frequently. The town of Calistoga was built as a spa resort in an area that has many natural hot springs. And where you have hot springs, you occasionally have geysers as well.
(An interesting anecdote. As the spa was being dedicated, its founder was supposed to christen it "Saratoga of California", naming it after another famous resort location. The trouble was, he was drunk out of his mind at the time, and he wound up christening it "Calistoga of Sarafornia" by accident. There is, of course, a restaurant in Calistoga named Sarafornia, as you can well imagine. That's one of the really cool things about this state: so many of the people who settled it were manly men, with lots of brawn and enthusiasm--but who were dumb as rocks. This theme seems to show up in our history quite a lot, actually....)
There's a petrified forest not too far from this spot. The petrified forest was primarily of California Redwoods (although I do remember one pine tree that had been preserved), which all got buried when a nearby volcano erupted. Geologists place the eruption about three and a half million years ago.
Of course, the thing that Napa County is really famous for, is all the wineries--which produce some of the finest wine in the world. Unfortunately, I can't recommend that for people who are traveling the state by car, with children, on a budget. The Wine Country attracts a very wealthy and sophisticated clientèle; expect prices to reflect this fact.
Of course, California has its characters too. A little further south, on the outskirts of Fresno, you can find the work of someone who was quite persistent (and also quite eccentric). A Sicilian immigrant, Baldassare Forestiere, moved to Fresno as a young man after spending some time digging subway tunnels in New York City. He bought up a decent-sized farm, then discovered that (A) the ground was really, really hard, and (B) it gets awfully hot in the summer in California's Central Valley. So he decided to build his estate... underground. For decades, he dug, and dug... and built quite a comfortable little place below ground, out of the heat. He discovered that fruit trees planted in a deep hole are easier to manage; you can pick the fruit from the highest branches just by walking up to it at ground level--and because it's cooler down there, you don't need to use as much water (which can get scarce here in the summer). There are some beautiful pictures of what this guy did at the link listed above.
And then there was Lady Winchester, wife of the guy that invented the (Winchester) repeating rifle. She became quite wealthy from the sale of these weapons--but felt very guilty at the bloody source of her wealth. She was quite superstitious, believing in the sort of spiritualism that was very common during the Victorian Era. She believed that her life would be claimed the moment that work stopped on her mansion. So she used up huge amounts of her wealth building a monstrosity of a mansion, with the work continuing, non-stop, day and night, for decades. This place is weird: it has doors that open onto blank walls, doors that open into nothingness (ten-foot drops), staircases that go up to the ceiling and then stop, hidden doors, one-way doors, trapdoors, rooms for seances, secret rooms.... It's the kind of place where they do their best business on Halloween, when they conduct flashlight tours. The Winchester Mystery House is located in San Jose, about an hour's drive south of San Francisco.
If you're willing to go a little farther south, there's Hearst Castle in the hills near San Luis Obispo--about halfway between San Francisco and Los Angeles. William Randolph Hearst was the Newspaper Magnate after whom Citizen Kane was modeled. He was ridiculously wealthy, and he liked to live the good life. Hearst Castle was built to be the centerpiece of a huge estate, which Hearst had stocked with exotic African wildlife. The Castle itself would have done Louis the Fourteenth proud. Hearst was also an art connoisseur, who packed his palace full of classical artwork--some of it dating back to the early Hellenic period. The whole place is gaudy and ostentatious--but it's also very, very beautiful.
I mentioned a few posts back about the air museum in Atwater, CA--the town where I lived during my high school years. That'll be right in the middle of a Central/Northern California excursion, if you're into Aerospace. But Sacramento also has a good air museum. It doesn't have the SR-71, and it doesn't have the big bombers like the Castle Air Museum does, but it does have a lot of good planes. I wrote a bit about our trip to that museum here.
Well! This is getting to be quite a long list. I haven't even gotten to all the beaches, or to Long Valley and Mono Lake, or to the Mendocino Coast, or the Monterey Bay Aquarium...
A few bits of advice: California is a big place--about 1000 miles from the Southeast corner, in the desert on the Mexican border, to the Northwest corner, on the coast at the Oregon border. There are huge variations in climate. Southern California is arid; Northern California can get a lot of rain. People have images of big sandy beaches, warm breezes... but that's Southern California. Especially in the spring and fall, weather in Northern California can be downright cold (especially if you go into the mountains), or miserably hot. Prepare for both. Remember Mark Twain's words: "The coldest winter I ever saw was the summer I spent in San Francisco." That is a true statement--it is not that much of an exaggeration. Be prepared for cold, damp, foggy conditions if you spend any time in The City.
But this is a beautiful state, and there is a whole lot to do here. I've barely scratched the surface, but this is more than enough for a few weeks' trip like what Chris will be doing with his family.
Oh, and Chris: let me know in the comments if you want to swing by our place. We're in a suburb of Sacramento, and I'd love to take some time off work and show you guys around this area. I need a bit of a vacation too, I think. :-) And it's been a while since my wife and I have set up a really good field trip for our kids.