Diamond X is a pretty cool course because of the terrain and scenery.
I also enjoyed Post Falls in Idaho but its nothing spectacular - just nice holes along a beautiful river.
Must plays? Eh, maybe maybe not but I definitely enjoyed playing at both of them.
We played about 12 or so more courses on our way out here from Virginia and those two were the best that we played along the route you are headed. Further east is a different story with about a million must play courses