A.L.
I actually vote for just walking around the downtown/"Magnificent Mile" area. There may be some specific art pieces in the gallery which you can't see elsewhere, and the Aquarium may be great, but when it comes down to it, you can find museums and aquariums anywhere. The beautiful old historic buildings and the feel of a city like Chicago, that you can't get anywhere else. So I suggest you head for N. Michigan Avenue and enjoy the view! It will be a bit chilly (wear layers and a couple pairs of socks), however if you take a break in a coffee shop, that can be part of the experience. Enjoy!