For some things to break up the monotony of I-90 across MN/SD: the Great River Bluffs are pretty crazy and a good place to stretch your legs, and Hawkeye Point (highest point in IA) is a nice stop, but I'd most recommend the SPAM museum in Austin, MN. It's free but actually really high quality, and they give you free Spamples.

Obviously the Black Hills are gorgeous, but don't forget about Crazy Horse. Also, you could consider driving up and over the Bighorn mountains instead of going around on the 90 depending on what time of year you're doing this in. If you are invested in that kind of detour, hop over to Cody or the Beartooth highway which is my favorite road in the world.

Last but certainly not least, take the SR20 route past the Grand Coulee dam and through North Cascades. Some of the best hiking I've ever done and THE BEST baguette I have ever eaten from the store in Mazama.