Millennium Park should absolutely be on your must-do summer list. And if you haven't made a trip to this mecca of all things free and fun, this weekend is a great time to do it.
From July 16th-18th, Target is sponsoring some special family activities as part of the Arts & Wonder Free Family Event. So you can take advantage of all of the typical fun stuff taking place at the park (like the Family Fun Festival, a street theater performance of Sarruga: Insects, cooling off in the Crown Fountain, exploring the Lurie Garden) plus you're in for some additional treats.
One last tip: Use public transportation to get there and save yourself the $25 or so that you would spend on parking at the Millennium Park Garage. Then I would use that extra cash to buy ice cream cones at the Park Grill for a special summer treat.