This one's easy: at the Lincoln Park Zoo, every day is a free day!
There can be a few costs, still: parking at the zoo, food, and a few rides. To avoid paying for parking, you can take public transportation, taxis, or ride shares. You can bring your own food and eat in an open space, under the Foreman Pavilion (seasonal, it's west of the Waterfowl Lagoon), at the picnic tables in front of the Park Place Cafe (these are more for people getting food at the cafe, but you should be fine eating there unless it's really busy—you can also use the south lawn by the tables), or in the Tadpole room at the lower level of the Park Place Cafe. And lastly, there are only a couple rides (the Train Adventure and the carousel) which are easy to skip. If you want to go on a ride, though, you can save a little money by buying in bulk. Instead of paying $3 for ticket, you can get 10 for the cost of 9 ($27) or 20 for the cost of 17 ($51).
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