We went out to Serpent Mound for an afternoon day trip. The mound was a bit smaller than any of us expected, but it was still quite impressive. We started at the visitor's center and then went to the tower to see the shape from above. You can visit the site in about 45 minutes, but there is also a nice, easy hiking trail that circles the site and drops into the ancient crater. That takes another 40 minutes or so, and was very do able by a six year old kid. One note... there is no real food places nearby, so if you go for the day, pack a lunch and eat at one of the picnic tables at the site. The gift shop and museum is not very interesting, but the docents there seem to know the history well. Parking is $7 and that's the only "entrance" fee.
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