St. Louis has some really nice riding! You can start at the Arch, there's a bike trail that heads North along the river and crosses the old Chain of Rocks bridge. This bridge used to be part of rt.66. This is one of 4 Mississippi river crossings that are bike-accessible. It's easy to connect to the Madison county, IL rails-to-trails, where there are probably over 100 miles of strictly bike trail. They run through tunnels under the major road crossings, and there are even bike on-ramps when two trails cross! Using different trails, you can pretty-much make any size loop you wish. The mississippi river trail also goes North even further, with bike trails about 50 miles further up-stream, along the Illinois river until you hit Pere Marquette State Park (nice campgrounds). With a few miles of on-road riding, you can hit the start of the Katy Trail, which essentially goes from St. Louis to Kansas City.
Those are just SOME of the BIKE TRAILS near St. Louis... We also have many streets with bike lanes! A bicyclist could go just about anywhere in town using the bike laned roads. It's really nice.
Of all these, my FAVORITE would probably be either the local Belleville, IL trail (because it's close to the house, and I use it a lot for practical purposes), The Madison county, IL trails (because the scenery is nice, and there is so much that you can't get bored of them), or the trail around Forest Park (because of a different kind of scenery... jogging college girls