The Katy Trail is "crushed gravel" or "crushed limestone" so it's not like you'd be riding on rocks or anything... It's fairly smooth, but I'd recommend something wider than 700x23 road tires. It's an ideal trail for hybrid bikes with smoother surfaced tires.

It's a beautiful trail along the Missouri River and goes all the way across the state, including the great Missouri wine country.

The suggesstion for Alton to Grafton on the Great River Road is a good one too... Used to ride that one a lot in my youth. That's along the Mississippi. If you ride it... take a quick detour through Elsah, which is a neat town tucked in the bluffs along the way.