Idaho
A state where a geological handbook [The Roadside Geology of Idaho] is a must. The south is extensive volcanic heaves, which may be related to the Yellowstone hot spot, and which I personally think may have been generated way, way back in time when an asteriod smacked the other side of the planet and caused a convergence of shockwaves in eastern Oregon and southern Idaho, but I'm no expert. I do know that a lot of geologists argue about why the basalt here, that reached truly monumental proportions in western Washington, like hundreds of feet thick.
You can dig jewels of various sort in Idaho, which I hope to do one of these days---this is a gift of that volcanic past. There are establishments that will let you dig, for a small fee. Bring your grubbies.
And one of the best hamburgers in the universe is in Horseshoe Bend, ID, at a little restaurant by the river just as you come into town. Huge hamburgers, and so good you wish you could stay for another meal. The portions are enormous, and worth starving for. Its address: The Riverside Restaurant, Hwy 55, Horseshoe Bend ID 83629, and the phone is 208-793-2651..."home of the half pound burger."
Beyond that, angled up toward Washington, is a route along the Payette River, which is gorgeous when the snow is melting. A little whitewater river that tumbles right beside the road for miles and miles and miles. Be prepared to take your time, but digesting those hamburgers could make it very nice indeed.
Up north is something called City of Rocks, which was one of the landmarks for early covered-wagon travelers, but we had a hilarious chase into the back country only to meet a "road closed" sign on a very muddy unimproved road, or cow trail. But we located a good Mexican restaurant, the Guadalahara. If you go City-of-Rocks trekking, don't miss the compensatory feast.