Feather River Canyon (CA route 70) is absolutely fascinating! it is a gorgeous canyon highway along a nice river, with straights, sweepers, and the occasional twisty section. For Rail enthusiasts, there is a rail line along the entire canyon, crossing under and over the highway bridges and river - intriguing (see photos). There are a few tunnels, many interesting bridges, and many places to stop for a swim if it´s warm out. I would highly recommend this route, and I´ll be back to ride it again!
Passing over Lake Oroville and round Flea Mountain, this scenic North-Eastern heading route is worth a look.