A day trip to Chicago took us down the Chicago River on an architectural tour to see the skyline, the buildings, bridges, etc. Seeing a city or place from the water is very different than walking on the ground. You learn so much from a different view.
We have done this architecture river tour dozens of times and each time we go, we learn something more about the history of Chicago. There are numerous tour companies, but we like the Chicago Cruise Line which has a little more intimate of a boat and tour (smaller and fewer tourists). Retired history professors provide the tours, so you get a mix of architectural information and history simultaneously. You can start lining up 30 minutes prior to boarding, which we strongly encourage getting there at least 20 minutes early so you can get a good seat.
Even if architecture is not your thing, just floating down the river, on a fall day, watching people, buildings, and bridges pass you by is a wonderful way to spend the afternoon. The tour/boat last about 90 minutes and consists of a Starbucks coffee, lemonade, tea, a bar, and snacks.
If you want something to do in Chicago and enjoy learning about history, this is a great tour! Easy to purchase online, get to (they board at the Ogden Slip, not the river), and it is on time and well organized. They also do Chicago history tours, Devil in the White City tours, evening cruises, etc.