Suggestions for a night and a day in Chicago?

I’ll be in Chicago with a college buddy for a night (say 8 p.m.) and part of the next day (until 2 p.m. when I’ll have to leave for the airport) from Sept 25–26. I’ll be staying downtown off Michigan Ave and Grand Ave and will have a car.

I don’t really know anything about Chicago, and I don’t really have a natural affinity for the city (like I don’t dream of traveling to Chicago someday to do all the cool stuff there). So I need some help.

What are some cool things to do that will fit the time?