gold panning in the Black Hills
Mammoth Site of Hot Springs, SD
wind cave national park
Union Pacific Steam shop tours--cheyenn
UP museum in Council Bluff Iowa
Porter Sculpture Park, Montrose, SD
Hiawatha Bike Trail in Idaho
Yellowstone NP
waterparks at Wisconsin Dells
Devil's Gulch Park Garretson SD
Minuteman Missile National Historic Site
Little Big Horn
Garnet Ghost Town
Devil's Tower--where ET the movie was filmed
All of these are interesting, just make sure you plan plenty of things for energetic kids. If they read up on the historical background, etc it will be a lot more interesting to them.
Look for some recommended books or movies about the region and at the very least read up on yourself.