There are a plethora of restaurants in Chicago that would be a wonderful "treat" for two parents away from the kids, but in order to help focus our suggestions, perhaps you could provide us with some more details about what you're looking for.
Do you have any particular "types" of foods you enjoy? Ethnic? Italian, French, Seafood, Steakhouse, etc?
In the same vein, do you have any dietary restrictions or aversions?
Do you have a particular location or neighborhood in mind? Will you be doing any other activities prior to/after dining such as shopping, theatre, etc? Would you like to be close to a highway, public transport?
What restaurants have you enjoyed in the past?
Answering these few questions can greatly help us answer your inquiry.