Speed limit on I-25 is 75 mph most of that stretch.

You will be on the plains, not in the mountains, for that drive. There's a little bit of elevation but not much.

You might consider taking 287 out of Ft. Collins to Laramie and I-80 to Cheyenne; that gives you a little more scenery, but it is a significant deviation from your route.

After you get to Gillette consider Devil's Tower National Monument - 60 miles to the east.