Honestly, I would not recommend going all the way downtown.
I have run into Chicago a lot, at least a dozen times so far, on layovers anywhere from 3.5 hours to all day. However, I will only do it if I have a minimum of 3.5 hours and then I will only go to Wicker Park. And the only reason I do it under these circumstances is because I know exactly where I am going in ORD to get to the L station and back, I already have my Chicago Card loaded with money for the L, and I know exactly where I am going in Wicker Park (mostly to shop, but I might grab some food to go), i.e. I do not need a map.
If my time whittles down to 3 hours because of unforeseen delays, I just will not do it. I will hang out in a lounge instead. I do not know much about restaurants around the airport, but that would be a much better idea.
BillJ