I just visited Chicago for the first time, and I really loved it! It has beautiful architecture, great food, friendly people, and it's both drivable and walkable, depending on your needs.
Here's how I spent twenty-four hours there. By the way, it was a beautiful August day, with unusually mild weather. I hear it can get quite steamy, windy, icy, sticky, slick, sultry, cold, or hot. Check the local weather before you go.