The Shedd Aquiarum is a fantastic place to spend a day. I highly recommend going as soon as they open in the morning to avoid large crowds and leave after lunch when it does get crowded.
The marine mammal presentation is wonderful and educational. The Lizards exhibit should not be missed, especially to see the Komodo Dragron!
There are always programs going on in the galleries on the main level for all ages and interpreters walking the floors to answer your questions.
The Wild Reef is amazing because of the numbers and varieties of fish in the tanks. There are some great touch screens there...