worked there for a week 15 years ago
if you are going april-oct you simply have to get to wrigley field, home of the chicago cubs, if they are playing when you are there
iconic stadium, really quite something different to experience and they are the current world series champions
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sears tower/willis tower skydeck is a must too
Cubs was the first thing I looked for, seems the dates I have free they are playing away unfortunately, will check the others in due course.