If I'm staying a couple days to ride Going To the Sun and Lake Koocanusa/MT-37 is it best to stay in Kalispell? Whitefish? I'll be staying in a hotel, not camping. I'll be coming up from the south and then over Lolo pass.
Your day 2 route would be my choice also: curves, rivers and mtn passes. Going west from Stanley, traffic will be light and the road good along rivers. Once you get to Banks, ID (a non-town) and turn north on also curvey Rte 55 you'll be on the river side of a spectactular whitewater river with plenty of turnouts for miles. This road can have a lot of traffic by Idaho standards, between the cars and the beautifully heavy rapids to look at (during summer water release from dam; I used to whitewater kayak) I have a hard time doing corner carving here.Day 2 I was going to go through Ketchum to Grangerville.
Day 3 over Lolo Pass and up to Whitefish.
Then ride the Butler gold roads in the area for a couple of days.