I lived in Couer d'Alene for 10 years and have flown that route many times in a 250. The I90 route is very good. Billing throught the pass at Livingston to Bozeman. Follow the valley to Missoula and then over Mullan Pass to Coeur d"Alene. From Coeur d"Alene Over Spokane to Ellensburg, Wa. Then over Snoqualmi Pass to Seattle.
Beautiful trip. The terain is not too challenging, but it is high (6000 - 9000) over the passes. IFR not recomended without oxygen and some local knowledge. Anytime from mid Julty through September should have good weather.
Jim Hiatt