Fifth wheel Travel on I-90 from Montana to seattle

All of the routes are quite scenic but stay out of Seattle unless you need to be there. US 2 is an easy drive although traffic gets heavy as it approaches I-5. 12 is not a difficult trip either.

You may consider WA 20 for return trip east out of Mt Vernon, WA
Or US 2 east out of Everett, WA.
Both are good two lane highways.
Could take US 12 all the ay west to I-5

Certainly consider the route that enblethen suggested, but I-90 is also a pretty drive......though, it is still an interstate! The Lolo route would be a great choice.....maybe I-90 for the return trip.

West out of Missoula, I would drop down on to US 12. It is a good two lane highway. Yes it has somewhat of a steep decent from Lolo Pass. It is not bad if you are aware of it. Follows river down the canyon to Lewiston/Clarkston
Another of our favorites is WA 821. It takes off near Selah, WA and follows the Yakima River. Some great BLM campgrounds along it.