Annett Nature Center in Warren County has several hiking trails, an observation tower and pond. The trails aren't marked, so I recommend printing or downloading the trail map ahead of time because my cell reception was very poor there. One of my kiddos led our hike and did a great job following the map.
The observation tower provides a great view and is definitely worth climbing.
After our hike, my boys enjoyed participating in the annual Mud Day event, which happens each year at the end of June.