If you're heading for southern Idaho and the Snake River valley, there is no better sight than Bruneau Canyon. Even though it is only slightly less spectacular than the Grand Canyon, everyone I know who has seen it (and that's not many) has seen it in solitude. The road into it is unpaved and traverses an old(?) Air Force weapons range. You may not even be able to negotiate it in February, but if you can it is well worth it. As an added bonus, the Bruneau dune fields are in the same area - take the climb to the top of one.
If you're heading for northern Idaho, just be aware that it has perhaps more scenic byways than anywhere else. I've driven just a few of these, but those were full of hidden gems, and at this time of year, you might have these roads essentially to yourself.
AZBuck