Narrowing down the choices is always the tough part of a trip like this, there is so much you can see, yet a limited time to see it.

As far as finding the small towns with history, sometimes your best bet is just to get off the Interstate and talk to people in the towns you drive through.

I suggest you read one traveler's trip through South Dakota as an example of this. While the Dakotas could be a great place to find it, there are countless other places and routes where you could find a trip that is just as enjoyable.