I don't think these parks are really all that secret, but that is how they were billed when I was younger. Regardless, they seem to be off the path (pun REALLY intended) for people like me because I don't make enough of an effort when it comes to entertaining my kid. And if none of them are new to you, I apologize in advance (I am prepared for a proper schooling once I hit "publish").
a.If you happen to find yourself in the ladies room on the 96th floor of the John Hancock marveling the view (and what a lovely view it is), look straight down if you can stomach it and you will see a little park tucked in between Michigan Avenue and the Museum of Contemporary Art. Maybe you have already hit up the Seneca Playlot, but if not, it's a nice little stop in the heart of the Magnificent Mile. Plus it's next to the Chicago Office of Tourism building-thing, so you can grab a coffee or stop in for a restroom break and go back to pretending to hang out with your kid(s). And if you are visiting someone just a few steps away at Northwestern Hospital, it is the perfect fresh-air-break spot.
b. We recently moved from the Wicker Park neighborhood to Old Irving Park. While Wicker Park is a fabulous park unto itself (Saturday musicians, soft ground for your kid to fall flat onto his face, hot dads), Dean Park, located on a dead-end barely west of the Jewel (1300 block of Ashland), often sports a smaller crowd, a great sized green-space and up-to-date play equipment. Plus, once the weather warm up, it turns into a splash park.
c. In Irving Park, we stumbled upon Kolmar Playlot, a sweet and clean little playground that caters to kids 12 and under. Plus no drug deals going down (that we witnessed) like our old 'hood. What more can a kid ask for? Unless he wants crack. Then he could ask for that.
There are a lot more...but as I already stated, I am too lazy to find them. So please send suggestions our way! Thanks.