There seem to be almost as many restaurants as people in Chicago, so there's no shortage of places to go and eat in the evening, and these range from bargain basement pizza houses on the street where you can get a slice cheaply at any time of day or night all the way through to gourmet establishments that cater for really special occasions.
Highly recommended is The Signature Room at the 95th (on the 95th floor of the John Hancock Building), which offers awesome views of the city, and also great American cuisine. It is vitally important to reserve a table in advance at this popular restaurant, and expect to applaud at least once, as almost every Chicagoan seems to propose marriage in the restaurant.
