Idaho rocks. Yellowstone- crowded, touristy....though popular, I had to move on.

Sawtooth Mountains were incredible. Glacier is absolutely phenomenal.






no longer own this bike





Going to the Sun Road, Glacier National Park, MT

 

my brother and I have rode our ol Harleys over most of the United States and the Lolo pass is one of the beat

 

As others have said Craters of the Moon is awesome. Different than you'll find anywhere.
Any highway is a safe bet really. If you need to cut back on time hop on the freeway. It's some of the same scenery, but highways are always king. Once you are outside of Treasure Valley (Boise) the speed limit is 85.
The further north you go the more pine trees you'll find. Southern is more sagebrush.
I'm not much for Sun Valley myself, not really my vibe but lots of people like that area. Sightings of actors and such. LOL
If you can get more towards the middle Hwy 95 is a good road. You can follow it up towards Couer d'Alene which I like. Loved climbing White Bird. Even the stretch of freeway that goes between CDA & Wallace is beautiful. CDA is another touristy town. Prices go up around there.
McCall is a nice little tourist trap. Not too pricey as it isn't as popular as some of the other tourist traps. Hwy 55 and/or 95 will get you there. If you only have time for one way choose 95. Little more scenic but both are good.
Towns are pretty safe everywhere. All cities are small here. Even Boise.

Some of your "larger" areas are Twin Falls, CDA, Idaho Falls, McCall, & of course Boise.

 

If you do get all the way to Boise and you like curves, head up to Bogus Basin. Short trip with I believe over 100 curves.

 

Bogus Basin one of the best places I have ever skied. And cold SOB when I was last their.
Boise has a great night life down town and tubing the river in the summer is a good time also.

 

Cold in the winter and hot in the summer!

Tubing the river is only fun if you like topless women....which I do!!

 

I can't tell by any of the responses if you are going up to Glacier, but yes it is beyond "worth it". Yellowstone is great for the animals and geysers but honestly, in my opinion, not as phenomenally scenic like Glacier. So, go to Glacier and if you come up from Yellowstone, start on the east side at St Mary and go west. When you leave the park, continue on US 2 to Whitefish. Rather than taking US 95 south through Flathead Valley, go on MT 35 to MT83 down to Seely Lake. This long valley is fantastic, compared to the other side which is Flathead. MT 83 ends east of Missoula, so head west to Missoula on MT200. All are excellent roads.

Once in Missoula, get on US93 south. You can cut east at Salmon ID and get over the Idaho Falls quickly. If you continue down US93, at Chalis, you can either stay on US93 or go southwest on ID75 into Stanley. From Stanley, head southeast on ID75 to Stanton Crossing and head east on US 26 to Craters of the Moon and back into Idaho Falls. From Idaho Falls, Swan Valley to Jackson to the south entrance to Yellowstone is a good way to get back. I have done all of the above and more and this has the best scenery and good roads. There are faster routes but as usual, the faster, the uglier.