Considering you're planning a road tour, I can offer a couple things to consider:
A couple reasons don't have anything to do with our state: But if you missed Yellowstone NP, Glacier NP, or the Grand Tetons / Jackson Hole WY, then you haven't planned a proper tour. These are fantastic places not to be missed, and I would include one of these at least into your plan. And if you do, you are most likely going to drive through Idaho to enter or exit these areas. If you're coming from WA state to see a number of things there that are must-see's, then you would have to drive through Idaho's pan handle to get to Glacier NP. In which case you would want to see northern ID as mentioned earlier. If you are heading to southern Yellowstone or the Tetons/Jackson Hole, then you will pass through Boise (at least check out the Capitol Building). On the way to Yellowstone/Tetons you can stop and see Shoshone Falls (the NW's Niagara) and Craters of the Moon. Both pretty cool.