Heading back from a day-trip to Muncie while visiting grandparents, I stopped to have the kids expend some energy at Mounds State Park, which is home to one of neatest Native Indian mound sites.
The main site is a large ring with smaller mounds that line up with the equinox and solstices. You can access the center of it, but you have to stay on the path to help preserve the site.
You can also hike along the river on several trails that even younger kids can handle. Looks like a nice little place to go camping, too.
I'd been to Mounds SP as a kid, but I don't remember this nature center. It's either new or improved or my memory is shoddy.