Carhenge
in Alliance is a really great monument and satirical version of England's Stonehenge. It is completely made of real cars.
Also don't miss Chimney Rock National Historic Site in Morrill County.
No trip to this region would be complete without stopping at the Toadstool Geologic Park located in the Oglala National Grasslands northwest of Chadron.
Agate Fossil Beds National Monument
301 River Road, Harrison
+1 308 668-2211
Free
Daily dawn until dusk; Visitor Center: Memorial Day-Labor Day - Daily 8AM-6PM; Off Season - Daily 8AM-4PM; Closed Christmas, New Years, and Thanksgiving
Fossils of ancient mammals, Native American history, hiking. There is no camping at the monument.