While we're waiting for Passepartout to show up, I can offer few things:
South of Kalispell is Flathead Lake. It's big. There are recreational activities on the lake.
South of the lake is the small town of St. Ignatius. Just west of there is the National Bison Range, part of the National Wildlife Refuge system. It's a great drive-through experience, and may present a nice up-close experience with the resident bison herd, Pronghorn, maybe even elk.
https://en.wikipedia.org/wiki/National_Bison_Range
Missoula, Montana is a nice town to spend time exploring. There's a Costco there, if you need one. (There's one in Kalispell, too, but Missoula is bigger.)
If it were me, from Missoula I'd head east on I-90 to Livingston, Montana, then head south to enter Yellowstone National Park from the north end. Drive down through Yellowstone as far as Old Faithful, then head back up and out the west entrance to West Yellowstone, Montana.
From West Yellowstone you can head west to catch I-15 at Idaho Falls. The Falls are pretty cool to see.
Heading south on I-15, you'll enter Utah and drive down through the Salt Lake Valley. Seeing the Great Salt Lake is pretty interesting. Salt Lake City itself is also interesting, as is a visit to Temple Square. If you happen to be there on a Thursday, (might be Wednesday, I forget), the Mormon Tabernacle Choir has a rehearsal that is open to the public. Very nice to hear.
Further south in Utah on I-15 you start getting into my second favorite place in the world to visit. (Right after Hawaii.) Bryce Canyon National park isn't far east of the freeway. Further south, Zion National Park is also not far east of the freeway.
After Zion, you soon arrive in St. George, Utah, a great small town that's a nice place to stop.
From there, it's only about two hours on the freeway to Las Vegas.
Hope this gives you some ideas. (And notice I didn't say a lot about Idaho stuff. Saving Jim some space for his thoughts. And now I see he just posted above me. I knew he'd have great ideas.)
Dave