On a budget, weigh it — Grant Park isn't cheap for what it is.
Free
A vast, open lakefront park that's the stage for the city's biggest events and a fine place to lie on the grass with the skyline behind you. The catch is that as a day-to-day destination it's somewhat featureless — its best moments are during festivals, and the more designed corners (Millennium, Maggie Daley, Buckingham) are what actually pull visitors.