Like most people around Chicago, I knew of Lincoln Park Zoo (free, but difficult to get to from the 'burbs and parking is expensive). I knew of the Brookfield Zoo (easier to get to, but you pay for parking and admission). I never knew there was a zoo in Wheaton until a couple weeks ago.
The Cosley Zoo, 1356 N Gary Ave., Wheaton, is small, pretty and FREE. Yep. Free. They do accept donations, so if you feel compelled to give a few dollars they will gladly take it.
It's not very big, and it's not very fancy. But it is nicely maintained, with lots of pretty trees and some cool animals. I believe they're all North American -- a rare cream draft horse, various birds of prey, turtles, peacocks, foxes, a coyote, chickens and cows and what not. I liked seeing the owls -- some of them are tiny!
There's a picnic area, and a snack bar/gift shop. Other than that, it's just a nice little walk through the park to look at and read about various animals, plus educational activities for little ones. I would say best for kids 10 and under, though older kids might be entertained for a half hour or so. Which really, you don't need more than an hour, maybe 90 minutes here, unless the kid in your life really loves goats or something.
Still, it's quite pretty, easy to find, and a nice way to kill some time on a warm afternoon in the 'burbs.