Yet another Yosemite vote.
The summer crowds will have somewhat abated by Sept. (but there will still be a lot of people), and the falls will have much less flow than their spring torrent (smaller ones will have disappeared completely), but even with those two seeming strikes, Yosemite is an absolute must if you have never seen it, or even if you have already seen it. Also, make sure to take a side trip to Mariposa Grove in the far south of the park to see gargantuan trees. As mentioned by others, you can then drive past Tuolumne Meadows, Mono Lake, and then blow all your money with Tony in South Lake Tahoe.

Nothing against the Pacific Coast or Big Sur, but you need to see Yosemite before California falls off into the ocean. Besides, there'll be cool new coastline to explore at that point.