There is one in southern Illinois called Tour de Shawnee, that is a good ride. Nice hills, great scenery and in the fall so it isn't too hot. I don't know how many it attracts but there are enough riders that you never feel you are riding alone.
Another one in East Prarie Missouri called Tour de Corn. This one is flat, even the century, but it has good rest stops and is a fun ride. Attracts about 800 riders.