I must second Mission to Phoenix, to Las Vegas to Reno, up 395 but then I would cut over to 97 on the 31 in Oregon or via Adin to Klamath Falls in Calif but there is a bit of mountain going that way..
I took 31 few years ago and I don't remember any mountains, but little to see or do. (I also took 395 thru Oregon and did hit the mountains near John Day but what a beautiful place to spend some time.) I'd stay on Hwy 97 to I84 west and then take I205 across the Columbia avoiding Portland.
You could also go on the I10 to the I5 but in Northern Calif and southern Oregon you'll hit the Siskiyous and while they are not bad, they are mountainous and curve.