The Petrified Forest is pretty cool, as is the Old Faithful geyser in Callistoga. But you can find camera-worthy subject matter almost anywhere if you're willing to get off the main highways like 12 and explore a bit. 121 from Napa to Lake Berryessa is pretty scenic, but many of the good vista points have no place to park; you'll need an accomplice to drop you off, and come back when you're done shooting. Get a good map, and explore the back roads. If you want to get adventurous, get a 4WD vehicle and explore the trails between Lake Berryessa and Clear Lake near Rumsey:
It's rugged, but there's some awesome vistas there. You might wnat to get a good GPS with map. The gray lines are 2-track trails; Hwy 16 is the only road likely to show up on a map. The trail to the point marked "excellent sunset vista" leaves Hwy 16 at a park; it's kind of hard to find, but well worth the effort. Here's a sample of what you can find:
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