I just did this in March.
A few must sees:
Russell's Travel Center and Car Museum on I-40 in New Mexico, just before you get to the Texas State Line.
https://www.russellsttc.com/CanyonAg77 was kind enough to give me a great tour of Palo Duro State Park. Seriously, I had no idea that such an amazing canyon existed in Texas.
Just west of Sturgis, Mt. Rushmore, etc., is Devil's Tower, Wyoming. That was a great visit. My 22 year old daughter loved the Prairie Dog Town.
Just south of Mt. Rushmore is the Crazy Horse Memorial. This thing is insanely large and they've been working on it for decades. And given the vision, they'll be working on it for another 75 years I'd say.
https://en.wikipedia.org/wiki/Crazy_Horse_MemorialIt's a little pricey but, it's worth it. I didn't do the bus up close to the memorial but the thing is big enough, you can see it pretty well from 2 miles away.
Depending on your route, there's plenty to see in Nebraska, Kansas, Colorado.
I stopped at the Eisenhower Presidential Library in Abilene, KS. I also toured "The Allen House" - a Frank Lloyd Wright house built in 1915 in Wichita.
Omaha allegedly has one of the best Zoos in the world.