DH and I are travelling to these 2 states this summer--they are the only 2 I haven't visited in the US. We are going while the kids are at camp, so we only have 4 days total including flight days. Thinking of flying into Missoula and then driving over to ID. Good idea or bad idea? 

Bad. Two days is no time at all. 

Bad. Two days is no time at all.

Agree. Pick one or the other and restrict your travel to a small part of that.

Both states are far too beautiful to just give a passing glance. 

Oh I guess I didn't explain that very well. I don't really want your opinion about going--we ARE going and only have 4 days. I was asking about the Missoula--ID part. What are the best parts of states to see? 

Spend your time at Yellowstone, Jackson Hole, Cody, area. So much to do there and your days will be filled with lots of interesting stuff. 

Yea, 4 days would be tough. If you fly to Missoula I would spend all my time around Flathead and Glacier - Awesome!

Another option would be flying into Bozeman and see the northern part of Yellowstone, or

Fly into Jacksonhole, spend one day in southern Yellowstone and one day to drive over the Teton pass to ID side.

Either way I hope you have a blast. Can't wait to get back myself. The Canadian Rockies are even a bit better around Banff! 

You could do 4 days at Glacier too. 

Central and northern Idaho are both beautiful (native Idahoan here). For your Idaho part of the trip, I'd go to Couerd'alene and north to Sandpoint & Bonners Ferry. If you have the time, St. Maries and Moscow are pretty too. Mostly a scenery trip. 

Go down I90 from Missoula to Idaho and drive around the Coeur d'Alene river. Its only 2 hours away. Its great for a day of fishing, barbeque and a slow rafting. The waters so clear you can see the fish. Or maybe take a logging road up into the mountains and hike around the area. The nice thing about Idaho is there are way more road than around here, and they take you where you would logically expect them to, you just point in a direction you want to go and there will be nearby road to take you there for exploring.

CDA lake is 3 hours away, but I would only go if your going to rent a boat/jetskis or something. Glacier national park is 3 hours to the north in a sorta opposite direction.

Or you might want to head south on 93 and stop by lake Como maybe look for an off road up into the Selway Bitterroot mountains (the jagged mountains to the west of 93) - very scenic area to hike. make sure you get gas every time you see it, or you might run out. bitterroot

Yellowstone park is over 4 hours away. Not worth it in my opinion. 

^^^what dog said!