I would hit the Reptile Garden in South Dakota, I went there as a kid and will always have a soft spot for it. Also, in my opinion, it is absolutely worth it to do the Grand Teton National Park if you are going all the way to Yellowstone. I would also recommend driving the Beartooth Hwy up into Montana.

On the way out, must see’s, in my opinion, would be Scott’s bluff in Nebraska, and Ft. Laramie and Independence Rock in Wyoming. Independence Rock was a stop for all the wagon trains headed West on the Oregon Trail, and they all wanted to reach it by the 4th of July which would insure them getting over the mountains before Winter. This rock was over a hundred feet in height and about 600 feet long. Many of those pioneers would climb up the rock, a not difficult job, and etch their names and dates into the rock, where they may still be seen. There are several other places of historical interest, including an old ranch, now restored that was a stop for the Mormons traveling West (this was also part of the Mormon Trail) that you can visit. So allow a little more time going out. I’ve been to Yellowstone many times, it’s my favorite vacation destination of all time, so make sure you get to all the little destination points on your little park map that you will get when you enter, in particular Old Faithful Inn, which is right next to the geyser, and the paint pots. Drive slow and keep your eyes open and you will see herds of antelope, deer, elk, and buffalo, plus wolves, coyotes and maybe bear. Some of these herds may be crossing as you travel, so you’ll have to stop to let them cross. When you leave the park, take the West entrance to exit and be sure to stop and see Devil’s Tower, it’s only a few miles off your route as you head to the Black Hills and, of course, Mt. Rushmore. After you’ve left Mt. Rushmore and you’re eastbound on I-90, be sure to stop for a cup of coffee at Wall Drug , Wall, S.D. and Custer State Park. Enjoy your vacation to remember!

I would recommend the trails around Devil's Tower. It won't be far out of the way.

In South Dakota the Badlands and Mt Rushmore, in Wyoming the Devil's Tower and Grand Tetons,

In S. D. you should take a drive through the Badlands then the Black Hills, Mt. Rushmore and the Crazy Horse Memorial. Devils Tower in WY is also not to far off your route.

You'll be driving through just a bit north of where I live.

But I'll be in Boston from the 17th-24th

I'm reliably informed that US Bank Stadium in Minneapolis is worth a look and of course keep an eye out for Yogi and Boo Boo

Black Hills of South Dakota? Deadwood? Gran Tetons?

On your alternate route you'd hit KC which has lots of cool stuff, although Chicago has lots of cool stuff too. If you have time, the River Cruise in Chicago is amazing.