Drive north from Spearfish, SD. If your vehicle is up to driving a long way on dirt roads, and you're a geography nerd like me, stop by the Geographic Center of the United States, in a field in the middle of nowhere. To be fair, the dirt roads are quite good quality, and it really didn't hurt my drive time by that much.
As you head north into North Dakota, the landscape changes... the mesas become sparser, small islands in an endless rolling sea of rich green grass. Once you arrive at I-94, just a few miles' drive west will bring you to Theodore Roosevelt National Park. The arrival is extremely dramatic; the land just drops away suddenly from the side of the road. There are actually two separate areas to see in the park. The first one you come to is Painted Canyon, with a visitor center and a relatively short hiking loop; the second is the longer driving loop a little further west, opposite the small town of Medora.
The driving loop consists of East River Road & Scenic Loop Drive and takes about an hour for the complete loop. I made it about 3/4 of the way around (clockwise) before being forced to turn around by a washed-out road (the rangers had advised me of the situation at the entrance). Along the road are numerous pullouts, dramatic bluffs overlooking the river, short hiking trails, and sweeping vistas. The scenery is interesting. Some of it is dissected and alien-looking like the Badlands; some is more rounded and gentle, though stark. It honestly reminded me quite a bit of Palo Duro Canyon in Texas. Keep an eye out for buffalo and wild horses!
The chief attraction in Medora is the Medora Musical, which runs in an outdoor amphitheatre in the evenings and has an optional "Pitchfork Steak Fondue dinner" beforehand. It's fun I suppose, but fairly cheesy, and quite expensive (I think I paid $100 for dinner and the show), but... it is "the thing to do" when you're in Medora, so I went ahead and did it. It's not really the kind of thing I would normally spend that much money on, but I indulged this once. For a family, it could get prohibitively expensive.
I spent a very comfortable night at the Amble Inn, which is unusual in that it has a large front-of-house bookstore and gift shop with stuff all about the region and North Dakota. This was not merely a generic motel; the rooms were really quite nice and comfortable.
Medora is a cute town, very small and therefore easily walkable. Lots of Old West-era buildings, and the town has a very specific history featuring many colorful characters including Theodore Roosevelt himself. Be sure to visit Roosevelt's Maltese Cross Cabin (which is in town, but on the grounds of the National Park) and Chimney Park, which contains the ruins of the Marquis de Mores's historic meat-packing plant. On your way out of town, tour the Chateau de Mores, on a hill overlooking the valley. Like I said, lots of history.
I chose to take Old Highway 10, which parallels I-94, rather than the interstate itself, all the way to Beach, almost on the Montana state line. You're not pressed for time on this day, and if I have time I always enjoy the calmer pace, lack of traffic, and small towns on back roads. At Beach, you have no choice but to get on the interstate, but you won't be on for long; cross into Montana and get off at the first town you come to, Wibaux. This is the northernmost point of your entire trip, halfway up Montana: from here it's equidistant south to Wyoming or north to Saskatchewan. Head south on State Highway 7 to Ekalaka and then 323 almost all the way to the state line. This is beautiful but lonely country; Baker and Ekalaka are practically the only towns you'll encounter on the 2+ hour drive south to Wyoming.
Once in Wyoming, you'll quickly start climbing back up into piney hills; these are the Bear Lodge Mountains, technically part of the Black Hills. You can take a very brief detour to drive around the tiny but cute town of Hulett; I drove up on a ridge there from which I caught my first glimpse of Devil's Tower to the southwest. Continue on to Devil's Tower itself, and use your Parks Pass to get in (I told you it would pay for itself!).
Devil's Tower is both magnificently impressive and mind-bogglingly strange. The famous columns (which range in number of sides from four to seven) are much larger than you think, and the debris field of fallen columns around the base is quite extensive. The Tower is curved slightly, and different sides of the Tower have different appearances, not only due to light, but also in and of themselves due to weathering. Park at the base (after checking out the prarie dog town just inside the entrance) and take a while to see the visitor center and walk the Tower Trail, which circumnavigates the base.
Lodging around Devil's Tower is generally limited to KOA and B&Bs, some several miles away. I chose to stay at Devil's Tower Lodge, by far the closest B&B to the Tower. I actually splurged on a room with a picture window view of the Tower, so that was literally the first thing I saw when I woke up the next morning. That was pricey, but definitely worth it. The rooms are extremely cozy and inviting, with log-cabin Western furniture and Native decor. One perk of this B&B is they include dinner as well as breakfast in the cost, so I had a great evening chatting with several fellow guests. Would definitely recommend, for all of the above reasons.