The terrain is pretty flat until you hit the Black Hills. So it's the river valleys that provide the most points of interest. The Minnesota River makes a slow loop from the SD border to Mpls, while the Red River meanders north along the ND border to Canada.
The Missouri River crosses the middle of SD, and has recreational dams, and L&C points of interest (including at the I90 crossing).
Most of the attractions on I90 seem to have arisen as a way of taking advantage of the tourist traffic headed to the Hills (and Yellowstone). Wall Drug is the most notorious example of this.
I'd check for state parks, and Corp of Engineers campgrounds. And look for roads the curve. There has to be a reason why they deviate from the normal grid pattern.
