If you drive it, plan to stay in McCall. Riggins is a cool town on the Salmon River. White Bird summit is kind of a pretty scenic overlook and has a cool history of a Nez Perce and cavalry battle. Lots of farms from there all the way to Lewiston and then on to Moscow. The Palouse is a pretty region. It would be nicer in May/June, but it's still cool to see in the fall.

Like many have said, you'd be money ahead to just fly into Spokane and run down to Pullman though.