MT/ID200 from the Missoula area to Sandpoint is fine. It follows a major river valley, without the passes the I90 crosses. Tourist facilities are less abundant than some other routes.
OR82 from La Grande is the easiest way to Wallowa Lake. Easiest way in, and easiest way out.
OR3 north to Lewiston has to cross the Grande Ronde at a low level, so grades in and out are long and curvy.
There is a paved Forest Service route around the east side of the mountains, connecting Joseph to Half Way, with a dam crossing of the Snake to ID71 (?). If you are used to mountain highway driving (e.g. like driving into the mountains north of Boise) it should be ok. Otherwise I advise going back around the west side.