No one has mentioned Mt. Sunflower?

What's wrong with you people! Wallace county, bring your O2. Interesting only for the novelty of it.
Arikaree Breaks in far, far, FAR northwest Cheyenne county is very cool.
Its the Saline River 15 miles north of Hays.
Monument Rocks (south of Oakley) has interesting rock formations. Castle Rock has lost its tallest spire, so its not the same if you'd seen it before. Castle Rock area has the better view still though.
Scott State Lake is something of a jewel in the middle of nowhere. Pueblo Indians lived there.
Old courthouse in Russell Springs in Logan county has (or at least used to have) a fairly interesting museum on the Butterfield Express stagecoach line, among other things. On your way there, eat at the Mexican cafe in Monument, if you're brave enough.
Don't drive I-70, it sucks once you get out of the Smoky Hills.