We live in northeastern Minnesota, and there's nice cycling up here, but if I had to pick one area in the Midwest within a day's drive of St. Louis or central Missouri, I'd pick western Wisconsin, particularly the driftless area of Wisconsin (roughly the area south and west of I-90). Lots of quiet paved roads, rail trails if you like them (including some mentioned above), good bicycling resources (such as the excellent Wisconsin bicycling maps from the Wisconsin DOT), interesting scenery with hills and valleys. plenty of facilities available, and generally bike friendly. And early October would be *gorgeous* there.

If you want to go farther afield, consider the area around the Keewenaw Peninsula of Michigan's Upper Peninsula. We recently went on a little trip there and it was great biking: http://www.crazyguyonabike.com/doc/?..._id=11056&v=88