Reply #13: The Shedd Aquarium is pretty fantastic

[Tyrone Slothrop]

as is the Adler Planetarium. (The Adler was the first planetarium in the Western Hemisphere and is the oldest in existence now.)

They're on the museum campus right by the Field Museum (which is also a must see -- where else are you going to see the largest T. Rex skeleton in the world???).

Of course, I'd also recommend all the other suggestions on this list. But one of my favorite things about Chicago is the awesome museums (the Art Institute is great too). In fact, I'd go so far as to say that Chicago has the best museums of any town in the country, my current home, NYC, included.