Chicago Loop (via Rockford, Madison, Milwaukee)
A long multi-day loop starting in Chicago and going through Rockford, Madison, and Milwaukee back to Chicago. In theory it would be mostly on trails, but construction season and optimistic definitions of what a trail is interfere. Still lots of fantastic trails! Milwaukee has some lovely biking, but getting in and out can be unpleasant.
A quick and easy day trip (one-way), outside of the usual transportation death doughnut on either end, where the city traffic is still high, but the bike options are minimal.
Might be a bit much for a day, but getting into Chicago (from this direction) is actually quite pleasant. The Crystal Lake Metra stop is very close to the trail, but you have to watch out for events that may close it down for bikes - and you miss some of the best parts that way.