Starved Rock State Park
This is probably the most famous State Park in IL – see http://www.starvedrockstatepark.org/history/ . We were there shortly after the major storm that devastated the park on June 30th to the extent that the majority of trails were closed. Only one trail – to French Canyon – was open near the Visitor Center, and consequently this canyon was pretty crowded. However, due to this trail closure, we decided to go to a remote Illinois Canyon (which we never visited before), and were happy that we did.