Southern Cal to N Idaho--Itinerary help

Ok, in a couple of weeks we need to drive to the Spokane/ Coeur d’Alene area, via laden (but not overburdened) 1985 Volvo 240 Turbo, with AC.
We are sick to death of 101 and I-5, and it turns out that all the family and friends en route are at Burning Man or away for Labor Day or something as we pass through, so [shrug]-- no reason to do that route again, and we’d rather do something with more interesting scenery.

I’ve never been to the greater southwest, so we were thinking of popping up via the I-15 and 90: LA to Las Vegas, SLC, and Butte, ad over to north Idaho. Mapquest puts this at only 2 hours longer than the 101–I-5–84–90 route.
So any input? Must-sees? How hot IS it down on the Nevada/Utah part of this itinerary? Don’t want to get mixed up in the Yellowstone thing-- we’ll mostly just drive drive.

I’ve done almost every mile of I-15 (from Sweetgrass, MT to LA) and I thought that it was just ok, as far as road trip interest stuff. I did it in Februrary, so I can’t comment on how hot you might get in NV/UT. Don’t know where your “stop for the night” stops might be located but try Primm, NV (I-15 on Stateline of CA/NV). It’s truly bizarre - giant hotel casinos all of a sudden in the middle of the desert. You can often get a really cheap room there, too (Try Whiskey Pete’s - we stayed for $17 a night, albeit in the off season).
As for Butte - C’dA - Spokane: Do it! gorgeous scenery. I’ve been to Couer d’Alene dozens of times in my life, but always entered it from the north (Cross border shopping trips in the early '90s, you see) from the east, it seems like an entirely different town. Very pretty on the east edge, by Lake Couer d’Alene.
Are you going to be staying with family or staying in hotels? (Just avoid the super 8 in Cd’A - nasty! run down! suprisingly expensive!) The Shiloh seemed to be a much better bet. (On Appleway @ US 95).
Let me know if there’s anything else you need to know about this particular neck of the woods, I’ve been there a few times!

What about taking 395 up along the Eastern Sierra Nevada? Nice scenery, and you have several options to make your jog east.