I have some family in southern Idaho and have driven through there on occasion. I highly recommend the Pie Hole in Boise when you're there. Good food, great prices, and it's open really late! Also, while I have never been there, it has been on my list for years: The City of Rocks National Preserve. It's only a couple of hours from Twin Falls and, I am told, well worth the extra miles!
Have a great trip!