In summer the central valley, although flat, could be unbearably hot. The sierra will be cooler but lots of hills. The coast will also be cooler but could be quite foggy.
And if traffic is a concern you might try farther to the North, where the population in California really thins out. Anywhere north of Redding is going to have relatively sparse traffic, and there are beautiful places to see up there, if nature is what you're looking for (nothing in terms of big city amenities). Here is one tour outfitter I found in the Mt. Shasta area but know nothing about. Good luck.
http://mtshastacycletours.com/