There are 12 properties and one former property listed on the National Register of Historic Places in Warren County. Information about each property can be found at this link on Wikipedia.
Map of Historic Properties

Coal Creek Bridge Hoosier Row School Indianola Carnegie Library Indianola High School Lake Ahquabi State Park Picnic Area United Presbyterian Church, Summerset Polled Hereford Breed Origin Site Octagon Barn, Otter Township Lake Ahquabi State Park Refectory Area Lake Ahquabi State Park Bathhouse Area Science Hall, Simpson College Palmyra Methodist Episcopal Church Former Designation: Warren County Courthouse, Demolished 2019