I'm woefully underexposed to Chicago, having set foot in the Windy City only briefly, and I look forward to exploring it, but for now, I will need to rely on friendly travelers, layovers and the like. In terms of food gifts from Chi-Town, hot dogs, deep dish pizza and Italian beef just don't travel well, so the good folks of Chicago came up with Garrett Popcorn. Garrett is a popular flavored popcorn shop that makes caramel and cheese popcorn. The caramel is good but pretty standard, while the cheese is fairly special, more greasy than powdery and full of cheesy-salty goodness. But the prize at Garrett is the Chicago mix, equal parts cheese and caramel corn (my daughter has dubbed it "charamel") which play off the sweet/salty flavors that we all find so enticing.
I have no idea if real Chicagoans eat Garrett popcorn or if it's simply a tourist thing (Chicagoans, please let me know), but the best thing about Garrett is that there are at least two at O'Hare (and they make the popcorn fresh on site).