There aren't wildly different temperatures from season to season in chicago but you may want to visit Spring or Autumn. April through May and between September and October, the temperatures are warm, and a variety of festivals take place and yet the crowds are manageable.
Baseball addicts may like April when the Cubs and the White Sox, typically host their respective home openers, There's brilliant public Art all through summer. The beginning of fall is the perfect time to go on a boat tour of the city (minus the summer crowds) or spend an afternoon walking through the Lincoln Park Zoo.