There are a bunch of places to see along the way. Kind of from the top:
Little Big Horn Battlefield National Monument, of course. It’s right off the highway.
There a whole bunch along I-25 in Wyoming that are off the highway a little ways. I’ll leave the details to people who live in Wyoming.
When you get to Colorado, I would suggest a side trip to Rocky Mountain National Park but it can be very busy. I would also suggest checking out some of the sites within the Denver Metro area. The Denver Museum of Nature and Science is worth a visit. Within Denver, there’s an alley art installation that I’ve submitted but was never accepted that is worth finding
A couple of cool restaurants to visit in Denver are Tocabe’s which serves Native American fry bread tacos. I suggest the shredded bison. If you are in the mood for Mexican, there are about a thousand restaurants that you can choose from but Chubby’s has the Denver Mexican hamburger which is unique to Denver.
I would suggest turning eastward as you go south rather than just follow I-25. The Callhan Paint Mines east of Colorado Springs are marvelous. From there travel south to La Junta and visit Bent’s Fort. You can also visit the Puragatory dinosaur tracks if you are up for a rather long hike (take lots of water). Go further east and visit Bent’s New Fort, the Sand Creek Massacre site, Lamar and the Granada Japanese Interment site. South out of Lamar you can go visit Picture Canyon petroglyph site.
From there I would back track to Capulin Volcano National Monument just outside of Raton. You are also going to be near Clayton, NM which has another dinosaur trackway that is easier to get to than the Purgatory site. It doesn’t have as many tracks but it also doesn’t have an 8 mile hike to go with it.