Near Iowa City, is the Herbert Hoover Presidential Library. On the West side of Iowa is the the Danish Emigrant Museum in Elk Horn. In NE is the Strategic Air Command Museum (about halfway between Omaha and Lincoln).
I much prefer heading into Yellowstone from the East taking I80 and then cutting up to I90 across Iowa and North East Wyoming and then heading over to Yellowstone from around Sheridan Wy (US 14). Much more scenic IMO. That would provide a stop in
Badlands of SD,
Mount Rushmore in Rapid City SD, and
Devils Tower in NE Wy.
I80 across WY is a desolote drive. You could cut up and over and stop at
Independance Rock on the Oregon Trail.