Serpent Mound is perhaps the most captivating of all of the native mounds that dot southeastern Ohio. The giant, uncoiling snake stretches over a quarter of a mile and is the largest effigy mound in the USA. You can walk around it or go up the observation tower for a sweeping view. The site is far flung, but cool enough to be worth the effort. It's 50 miles southeast of Chillicothe.
A small on-site museum (10am to 4pm Monday to Thursday, 9am to 5pm Friday to Sunday, reduced hours in winter) has a short video about the history of the mound builders and exhibits showing artifacts found in the area.