Obviously—for other posters—the best time to go to the Dakotas is in mid- or late summer when the sunflower fields are blooming. It’s easily one of the most beautiful sights I’ve ever seen and even more fun to know that the beauty is incidental to the cash crop (unlike Holland’s and California’s flower fields).
It seems like you enjoy Americana, so I would focus on doing things the locals do. Go to dive bars. Do some open-skating at an ice rink or go to a junior league or university hockey game. I am not sure if you’re single, but many of the women in those parts are descended from Scandinavians, and they’re beautiful.
I can’t wait to hear about the Amtrak journey in March. Please keep us updated!