Gibson County
I don't know much about this county. It has one nature preserve and a hunk of the Patoka River Wildlife Refuge. The Wabash Runs along one side, and part of the Patoka River forms another boundary -- a very squiggly one. The nature preserve is rated highly enough to be a National Natural Landmark, so it must be worth a visit. There are two national Wildlife Management Areas, also.
Patoka River National Wildlife Refuge
Cane Ridge National Wildlife Management Area
White River BottomsNational Wildlife Management Area