When will you be taking this trip? Time of year is important to know as some places will be pretty snowed in where mountains are concerned.
Some of my favorite places on road trips between Washington and Phoenix...
The rain forests on the Olympic Peninsula in Washington and the drive up to Hurricane Ridge
Mount Rainier National Park (depending on the time of year, because of snow)
Crater Lake (depending on the time of year, because of snow)
Southern Oregon coast/northern California coast and the Redwoods
Yosemite
If Tiago pass is still open and you exit Yosemite out that exit you can drive the 395 down stopping in Mammoth Lakes (take the gondola ride up Mammoth mountain, the views are crazy!) then continue through Bishop and Lone Pine, and you could go through Death Valley to get to Las Vegas.
Driving Hwy 1 through Big Sur in California is pretty epic, but may need to be saved for another time as it's on the central coast on the opposite side of California from Yosemite.
And if you're going to Phoenix (is that where you'll be flying back from??) for goodness sake go to Sedona! The Red Rocks are gorgeous! Take a Pink Jeep tour!
Hint: Go to a bookstore and buy or at least browse some DK eyewitness books. I love those when I travel and I also spend time at Tripadvisor.com when researching.
I'm jealous! Returned from a road trip on August 1st, and I'm ready for another! Lol!
p.s. It might be worth your while to look into a yearly pass called "The America the Beautiful Pass" if you're planning on seeing a few National Parks or Forests. We bought one this last road trip and it's already paid for itself, and we've got the rest of the year to go!
Have fun!