You crack me up! What a neat way you're traveling!!!
How about Badlands Nat'l Park/Rapid City/Custer State Park area (I-90) to Bozeman, Montana then head south and poke around Wyoming and Colorado mountains to return home. You could see Yellowstone, Grand Teton Nat'l Park, Mesa Verde Nat'l Park, CO; 145 north toward Montrose, CO and east on Hwy 50 to Colorado Springs. I-70 east to home or from Colorado Springs south into northern New Mexico and take I-40 east back home. I mention interstates but in same direction of interstates are many secondary roads to take instead - our favorite way.
Many times we've gotten to an intersection in a area of the country and said "right or left". That makes a fun trip! Safe travels.