I didn’t mean to trash the Interstate, except tongue in cheek. There is some interesting geology to be seen in eastern WA. When you crest Ryegrass (west of Vantage) you can get a killer view of Mt. Rainier.
Ellensburg is a nifty college town. The Cle Elum Bakery (est. 1906) will provide a torcetti (spelling?) which is sort of a shortbread twist that has provided my family with inner fuel since the early 1970s.
Snoqualmie Summit is pretty country.
The T-Mar cafe in North Bend was great but the divided highway bypassed the town.
Boehm’s Candy in Issaquah used to be a great stop when the highway was not divided.
I90 is fine. I put myself through U of Washington (the school that used to win all its football games. Not lately. Sob)working on building that hiway.
Still, US2 up through Leavenworth (as people have pointed out) is pretty nifty and has been for a long time.
I think a "great stretch of roads" thread would be interesting.