I know it's not exactly the normal beaten path, but I love the drive past Flat head lake and Clipping the West Glacier National Park Area.
Take I90 until you get Missoula, then Turn north on Hwy 93.
To get back to the main route you'll have to turn West when you get to Kalispell on Hwy 2 and go into Idaho that way.
You can turn south around Bonners Ferry to get back down to Coeur d'Alene and continue on I90 to Seattle and then down.
It's a 300 mile detour, but if you are looking for some scenery and some fun roads it's the way to go.
https://goo.gl/maps/518vBCM1z2K2
(That route has you going around the East side of Flathead Lake. I perfer going around the West side, but it's alot slower.)