In the early days of Chicago, you could get from the Gulf of Mexico all of the way to the outskirts of the city via water: first the Mississippi, and then the Illinois and Des Plaines rivers. But a rise in the land prevented any rivers from going all of the way to Lake Michigan, resulting in a stretch where goods being shipped via river had to be ported over the land. In the mid 1800s, the Illinois and Michigan Canal was built to allow shipping all of the way to Chicago and Lake Michigan, and today a bike path runs along two thirds of the path of the original canal.
By the late 1800s, new technology was starting to make the canal obsolete. The steam engine had been perfected, and railroads were taking over as the primary means for moving goods and people. Traffic along the canal dropped year by year, and the canal started falling into disrepair. Meanwhile, the much larger Sanitary and Shipping Canal was being built along a parallel route, making the I&M Canal redundant. Finally, the canal was officially closed in 1933 when the Illinois Waterway project provided an alternate path that could handle much larger and more modern shipping equipment.
Old transportation paths tend to make great multi-use recreational trails, and the I&M Canal is no exception. Today, there is a 62-mile crushed limestone path that runs from Channahon in the east to Peru in the west, following the original towpaths along the canal. Along the route, you can see much of the canal infrastructure still in place: locks, locktenders' houses, and aqueducts. While the entire 62 miles can be ridden, it is also easy to access the trail from any of the towns along its path or from a few other access points.
Starting at the highest point on the route in Channahon State Park, the main trail heads southwest. The first major sight is also one of the best: the Aux Sable aqueduct, lock, and locktender's house, which is about eight miles from Channahon. The next real stop is six miles further west in Morris, where Gebhard Woods makes a nice place to stop for a rest. More towns mark your progress as you continue west: Seneca after 10 miles, and Marseilles after another six miles. Each of these has their own sights and amenities. After crossing the Fox River Aqueduct, another six miles down the road, you'll end up riding through the edge of Buffalo Rock State Park six miles farther. From there, it's another 10 miles to the end of the trail at Peru.
While the trail can be busy when it passes through towns, the stretches between towns are generally devoid of other users. The trail's many access points make it easy to pick up and ride for shorter distances, but it also makes a great bikepacking trail: several on-trail campsites are easy to find on the western edge, and more are available in state parks that are near the trail. You can start planning your short or long trip at the Wireless Bike Map.