While in Chicago, you absolutely must see Grant Park.
The park spans more than 300 acres and offers quite a variety of things to see and do. Originally called Lake Park, it was later named for the 18th President of the United States - Ulysses S. Grant.
Millennium Park is at the top Northwest corner of Grant park, and includes varies art works and architecture such as the Cloud Gate and Crown Fountain.
After stopping by, then head to Buckingham Fountain which is one of the largest fountains in the world! It offers water shows every 20 minutes and functions from April-October.
Grant Park contains many other attractions, including museums, gardens and of course a dog park.