Hi again. I do recall reading about your plans last year.
I am not the only one on here who loves Montana and Idaho.... One of my favourite areas is central ID. The salmon river, the river of no return country, the Sawtooth Mountains and the scenic passes over the continental divide, especially Lemhi Pass. Approached from MT this is exactly what Lewis and Clark saw as they crossed the continent, thinking they may be at the coast and seeing all those mountain ranges yet to be crossed. That great drive was recommended to me by another member.
At Twin Falls there is the Shoshone Falls (the Niagara of the West), where you will see the canyon which Evil Knievel tried to jump on his motor cycle. And the Bridge across the same canyon north of town, over the Snake River. After 75 splits off 93, there is the ski resort of Sun Valley with the lovely little town of Ketchum and also Hailey. Along this road the lava fields come right up to the road, in places on both sides of the road.
On the way to the lovely mountain town of Stanley (pop 100) there is Galena pass with some great views over the surrounding countryside. In the vicinity of Sunbeam - itself a semi ghost town, there are several ghost towns, Bonanza comes to mind with its gold -history. One of the dredges is still there to see and climb on.
Just about every road in ID is a scenic road, especially the road from Stanley to Salmon, Loeman to Stanley and from Banks north along the river.
There is Craters of the Moon Nat. Mon and the Sawtooth Nat. Rec Area. Stanley Lake is a nice area to go for a walk (or swim) and there is a lovely resort at Red Lake with canoes, kayaks, etc. for hire. (If I recall correctly even small sail boats.) In the North in Sandpoint is the longest covered bridge.
Over the border into MT, just north of Missoula, at Moiese there is the National Bison Range - not to be missed. US93 from Missoula to Glacier National Park is a very scenic route, past the lake through Kalispell and Columbia Falls and Hungry Horse - not much more than a ranger station. I went in there and asked what there was to see, could they direct me to a place they would normally not recommend. That resulted in a hair raising drive up a mountain to a communication tower whence one could see into some of the glacial valleys of the NP a little further north. I will never forget that trip. Alas, not to be attempted in a rental vehicle.
I'll leave it at these few suggestions. There is so much to see in MT, I could fill a page with it. Do you still have maps or a road atlas from previous trips? If you look in the field reports you will be able to read some of the trips I, and other members have made up that way. One of the most recent posted is part of this trip.
Lifey