The Danish Windmill in Elk Horn is definitely worth a stop when you're driving between Des Moines and Omaha. We combined it with a visit to the Museum of Danish America. The 60-foot windmill was bilt in Denmark in 1848, shipped to Iowa in 1976 and rebuilt in Elk Horn.
The fee to go inside the windmill is $3 per adult, $1 per child. My kids enjoyed climbing the steep steps, seeing the Lego windmill and eating Danish kringle that we bought in the attached gift shop.