- Too many places to see/list, it's impossible to just put out a list
- Flat parts on the 5 usually are south of Mt. Shasta, and most of the central valley from roughly Stockton towards Grapevine
- Flat parts of the I-15 are past Victorville to Vegas, and Vegas up to Virgin River Gorge, and then from St. George UT all the way up into SLC.
- North of SLC, recommend you detour to see Jackson WY and swing by Yellowstone.
I have driven PCH from San Luis Obispo all the way up to the Oregon border. Highly recommend the stretch from OR down to SF if you can make it work.
As for what to expect, here's Jackson WY back to LA for reference:
[ame="https://www.youtube.com/watch?v=yKL7dvhPIFY"]Time lapse, Jackson WY to Los Angeles CA - YouTube[/ame]