I'll throw out a few things, and let you Google them yourselves. Or if you have questions about specifics, reply to this post.
First, Google Maps has you going from Brownwood to Cross Plains to Abilene. We prefer Brownwood to Santa Anna to Coleman to Abilene. Santa Anna is a quirky little town, the roads both routes are good, the Santa Anna route takes you into the south side of Abilene.
That route also makes it easy to go to Perini Ranch Steak House, which is a bucket list item, reservations encouraged. It's not exactly cheap, but the entrees are pretty big and they don't seem to mind folks sharing. Buffalo Gap itself is an interesting place, decent little museum.
However, if you go Cross Plains, there is a classic car and baseball museum (odd combo, I know) that might fit your interests. I've never stopped, but it is on my list.
https://www.woodysmuseum.com/I'm not a fan of Underwoods BBQ in Brownwood. Maybe it's better now, but a million years ago, when they had a location in Lubbock, it was pretty bland and fast-food level BBQ. Last trip, we tried Humphrey Pete's, it was a good, not quite great, stop.
From Abilene, I hate that stretch of I-20, I don't know why. I have often kicked north and hit 180, which parallels it, and joins US 84 in Snyder. This also gives you a chance to jog back east a little, and visit the ruins of Fort Phantom Hill.
But if you stay on I-20, there is a WASP Museum at the airport, which I have on my list to try.
From Abilene to Lubbock, not a lot to see, and several stretches of road with not much on them. Google Maps says to turn north at Slaton on FM400. I disagree. While it adds a few miles, things will go much smoother and quicker to stay on US84 to Lubbock, then pick up I-27. Fewer stops, lots more services available. Though the canyon crossing a few miles north of Slaton is fun.
Slaton attractions: Slaton Bakery, Harvey House, Air Museum (north on 400)
Lubbock, I love the Ranching Heritage Museum. Lots of old houses, barns, and other ranch buildings from all over Texas have been moved there. Also a special "Hank the Cowdog" exhibit for the younger ones.
One other route suggestion: Google Maps has as an alternate, going north from Abilene to Aspermont, Paducah, and Childress, then Amarillo. It's going to be pretty much rolling ranch country, few services. But if you like that sort of scenery, it's good.
However, and depending on your schedule, I deviate from Childress-Amarillo. At famous speed trap town Estelline, head west on 86 to Caprock Canyons State Park. A really cool park that few know about, and people get upset if I tell more people about it.
On the way to CC, Ozark Trail marker
https://maps.app.goo.gl/ZzQyTp67Zdiwse2F8 Turkey: Hotel Turkey, Bob Wills Museum. In Quitaque, the Comanchero Canyons Museum, only open on Saturdays, but sometimes a volunteer will come open it for you.
Caprock Canyon, wonderful park, much like Palo Duro Canyon, but less crowded, and less developed. Texas Bison herd roams freely. Attached is a rails-to-trails that's great. The Park also will offer van rides down the trail, that go to the Clarity Tunnel. Huge bat colony there, you can watch the evening flight.
If you like scenery and back roads, go north on highway 207 past Lake Mackenzie, and to the overlook at the lower Palo Duro Canyon. You can continue straight north to Claude, or backtrack a few miles and take back roads through Wayside to Canyon.
In Canyon, sad to report the PPH museum is currently closed. Fire and safety concerns in the 90-year-old buildings. But Palo Duro Canyon is open, the best state park in Texas, IMO. And the musical drama
Texas! will be going at that point. Another bucket list item.
Moving north, Boys Ranch has a little museum and a Boot HIll. Channing has the XIT business office, can't go in, but can look in the windows.
Clayton NM, obey the speed limits, which drop ridiculously low, way before town. Eklund Hotel is cool, don't know if they still run a restaurant there. Clayton Lake has a spectacular dinosaur trackway, only a few hundred yards from parking.
Nearby Capulin Volcano is cool, you can drive close to the top, and hike around the rim.
If you're willing to do back roads, you can see a lot of Santa Fe Trail swales north of Clayton, from the Cimarron Cutoff.
The drive from Clayton to Raton is pretty desolate, but there is a huge mesa to the north of US 87. If you are adventurous, cut off 87 at Des Moines, go to Folsom, and take highway 72 over the top of Johnson Mesa and into the back side of Raton. We think it is a beautiful drive, and it passes through the area of the Folsom Point discovery.
There is a spot west of Raton where you can see the K-T boundary, where the age of the dinosaurs ended. I think it's also visible north of Raton Pass, but I'm not sure where. Three to five miles north of Raton Pass, there are ruins of a church on a hill west of I-25. Very photogenic, was part of an old coal mining ghost town.
Depending on how much time you have, we love the loop around Spanish Peaks, as opposed to staying on I-25 from Trinidad to Walesnberg. Take highway 12 toward Stonewall, Cuchara, LaVeta, Walsenberg. The first part isn't much, but once you drive through the sandstone gap at Stonewall, it's really pretty. The volcanic dikes around Spanish Peaks are spectacular, LaVeta has a nice little museum.
As Hollywood said, not too far from Great Sand Dunes. Definitely a stop for the young folks. Hot, sunny, lots of walking through sand.
Back on I-25 to Colorado City. Here can divert to Bishop Castle (amazing) then Canon City. Canon City, drive over Skyline Drive across a sandstone hogback, look for dino tracks. Royal Gorge is nearby.
Lots to do in Colorado springs, Air Force Academy is usually a great stop, you can go to the visitor's center and field house, see several airplanes on sticks, watch glider and parachute ops. The famous chapel is being renovated, I think, so you might not be able to go in. The Class of 2029 will be there for Basic Cadet Training, so you might see their formations.
As mentioned by Hollywood, have paper copies of registration and insurance, you are subject to search, and don't even think of carrying a firearm onto a military installation.
Keep in mind, current geopolitics may lead to USAFA being closed to visitors entirely.
Not quite sure how to advise on Denver to Marshall. Google Maps seems to recommend just reversing your route Denver - Raton - Amarillo, then 287 to Wichita Falls and Dallas. If you do that, stop at the Charles Goodnight home, Goodnight, Texas, especially if you're a fan of
Lonesome Dove. I'm trying to figure out how Wichita, KS got into the discussion. But it looks like a route through Wichita, KS and Tulsa, OK, would not add a lot of distance or time. Some suggestions along those routes, especially if you do back roads.
Go to Pueblo, head east along Arkansas River. Bent's Old Fort at La Junta. Dinosaur trackway south of Los Animas, at Purgatorie, requires reservations, joining a caravan to the site, and a high clearance or 4WD vehicle.
Will second IKE museum in Abilene, Kansas, but we make a point of doing Presidential Libraries.
Dodge City, KS is a little hoky, but tons of Santa Fe Trail swales just west of there, Fort Dodge just East. Greenburg deep well and tornado recovery.
Further north, Kansas has interesting geology. We enjoyed Monument Rocks and Little Jerusalem Badlands. Guide to KS geologic sites:
http://www.kansastravel.org/chalkkansas.htmlComing down through Tulsa, your wife may make you go to Pawhuska for the
Pioneer Woman stuff. Go on to Bartlesville for Woolarock.
Okay, I think that's it, barring any request for more info. 