Some less-frequented-by-tourists spots:

Get off the main drag and take a drive through the mountains behind Santa Barbara (around Cachuma Lake) and/or the mountains behind San Diego (around Julian). Both are quiet, beautiful, geologically interesting, and "California as it used to be."

Inverness. Lives up to its namesake.

The drive on 50 to Tahoe is glorious, no matter what the season...stop frequently, get off the beaten track, don't just drive from SF to Tahoe.

The thing about California is that it's really a little country unto itself, climactically and geographically. You can be sweating in the desert and within a few hours' drive be bundling up with down parkas in the mountains.

My top 10 (in no particular order)have pretty much been mentioned:

Yosemite
Monterey/Carmel
Napa/Sonoma
Palm Springs Area
San Francisco
San Diego
Crescent City
Santa Barbara
Hwy 1 almost anywhere in CA
Marin Headlands (N. of GG Bridge)

San Fransisco has great weather in September --- California is fantastic -- I would concentrate on the National Parks and the coastal drives -- and I would skip Lake Tahoe, the mountains are not snow-covered in Sept....not a must see IMO.

Henry Cowell State Park (Felton, Santa Cruz County)... the redwood loop is beautiful any time of the year, but especially so in September.

Also - West Cliff Drive and the beaches of Santa Cruz. September is the finest time weather-wise.

"and I would skip Lake Tahoe, the mountains are not snow-covered in Sept....not a must see IMO."

IMO, September is a great time to visit Tahoe. The crowds are gone. The lake is swimmable and the skies are usually clear. If you like to hike, there are many great trails. And towards the end of September, the aspens turn to gold.

Tahoe is not only a treasure of California and Nevada, there is nothing quite like it anywhere else. It should have been made a National Park before the shoreline was over developed. But, you can still appreciate its beauty and enjoy it's grandeur if you get off the highways and on to the trails.

OBT, I absolutely agree. Tahoe is absolutely unique, imo. Even more so in the off-seasons.

Try http://www.panoramio.com/ & type in any of the scenic spots mentioned here (Big Sur, Yosemite, Lassen National Park, etc) & you'll get many great photos of these areas. Sort of like window shopping.

<What would your favorite time of the year be to go?>

Right now.

It’s green everywhere for about another month. The rest of the year is brown.

Stil....Fall is my favorite time of the year. Something of yester-year, harvest time, Indian summer along the coast.

I think you already have the most scenic picked out, although some shuffling required for time of the year.

For Sept, any of the wine trails, and coastal drives from Santa Barbara to the Oregon border.

Last weekend, just for the thrill of it, my cousin went snow skiing from 9am to noon, then at 1:45pm was at the beach doing some swimming and 82 degree sunbathing all afternoon.

All good recommendations. My favorites: Muir Woods, Napa, the drive to Carmel (through Monterey - just down the Coast - it's SO gorgeous), Yosemite.

Thanks everyone for taking the time to post your thoughtful responses. I am really enjoying googling all of these places for pictures. I have spent several hours researching them all. It's just going to be so difficult to narrow them down! I want to see it all.

Keep 'em coming...

Yosemite

Point Lobos

Coronado Island

Mendocino Coast

Sequioa National Park

We explored the coast from just south of Mendocino up all the way to Prairie Redwoods well to the North. Great, easy and very pretty short walks at such places as Fern Canyon, Mendocino's village and headlands, Russian Gulch. The Angelo Preserve, up in the hills east of the ocean, a bit North of Mendocino has wonderful day hiking among redwoods and douglas firs and other fantastic vegetation, hills and the Eel River. If you go up north past Eureka, you will drive through some wonderful redwood parks, with big trees right along the roadside (scenic route--not the main highway). Up North of Eureka and Trinidad is Patrick's Point State Park, perhaps our favorite place on the north coast. Even if you do not camp, day trip it. The campsites are in a grove of Sitka Spruce. There is a model Native American village there. Short trails go out from the wooded sites to cliffs on the ocean with spectacular views. Agate Beach, part of the state park is fantastic. Further North, we enjoyed a hike at Prairie Redwoods (Friendship Ridge Trail) which had us in the huge redwoods and firs on the hillsides above the ocean, along the beach and we saw Roosevelt Elk up close. Check the climate info though. In August, when we were there, it was foggy and cool right out on the ocean. We very much enjoyed the Shasta area as well. Castle Crags State Park just south of Mt Shasta was a favorite. The hike to the crags is amazing. In that vicinity there is great trout fishing on the upper Sacramento River. You can take your car up to about 7000 feet or higher on Mt. Shasta too (free!! when we were there a number of years ago). Hope this helps.