Brookfield zoo has these free days right now. Zoo Free Days Tuesdays, Thursdays, Saturdays, and Sundays from January 1 through February 29. They still charge $15/car for parking though. When my kids were little, we loved going to the zoo in winter. We parked at the South lot which has easier access to buildings. We would visit the Hamil Family Play Zoo, the swamp house, the penguin house and some times the ape house. We'd bring a lunch and eat it at the restaurant dining room next to the penguin house. We absolutely LOVED how not crowded the zoo is in winter.