The Chicago Architecture Foundation run fabulous tours around the city. We went on the boat tour. The tour guide was very funny, as well as knowing his stuff backwards. Next time I'm in Chicago I'd like to go on one of the walking tours.
In The Field Museum I came across an exhibit about the mound building Native American culture that flourished in what is now the Midwester, Eastern & Southeastern United States from approximately 800 CE to 1500 CD.
One of the most famous sites is the Cahokia Mounds, which Annie, Ziggy and I took in on the way to St Louis.
It's an astonishing place which originally had 120 mounds (there are 80 today) and a woodhenge.
(The mounds are obviously very tempting in the Winter!)
The Central Focus of the city is Monk's Mound, which is 92 ft high and covers just under 14 acres. It's the largest man-made mound North of Mexico.
It was horribly hot, so I didn't explore the sight as much as I'd have liked, but I did make it to the top of Monk's Mound.