Most of the best part of Chicago is walking around discovering what the city has to offer. I recommend good, very comfortable shoes as a required part of your dress while you're here. Chicago Cultural Center Wonderful free exhibits, lots of tourism information and a nice gift shop for those Chicago souvenirs you must have.
Crowds are usually larger on free days.
Besides the staple museums, etc., try the Lincoln Park Zoo . Zoo admission is free, there are plenty of places to stop and eat, feed the ducks. Then walk west on Armitage Avenue just a few blocks for Ben and Jerry's ice cream. Try the Chicago Reader for reviews. Stock Exchanges Chicago Board of Trade, Chicago Mercantile Exchange, Chicago Stock Exchange all have some tour options.