There is a lot to see and do in Chicago. It all depends on your choice of activities. Family, Fun, Adventure, Music, Sports, and so on.
For families, there are two zoos in Chicago and they are both great. One is the Lincoln Park Zoo, it's free and it's located near the lakefront. Navy Pier and Magnificent Mile along Michigan Avenue has some great shopping.
There are several museums that offer lots of exhibits. The Field Museum and The Shedd Aquarium are great attractions. Some of the museums have free days, just check their websites.
Downtown Chicago offers great architectural tours, visits to the Willis Tower (aka Sears Tower), riverboat rides, and great restaurants.
For the sports fan, visit any of Chicago's great sporting venues. Home of the Chicago Bulls, White Sox, Cubs, Blackhawks and Da Bears. Chicago is a sports fan playground.
For the music lover, visit Buddy Guy's Legends or Ravinia Park for summer outdoor concerts.
Personally, I love Chicago in the summertime when many events are held outdoors.