Chicago is a fantastic city alright; the only other US city I would rank as high is San Francisco.
If you are in Detroit, you are right on the Canadian border. Toronto & Niagara Falls are within easy driving distance (Detroit is about halfway between Chicago & Toronto). Also if you are at it, there are other parts of Ontario and even Quebec which are reachable; certainly worth visiting Montreal if you can.
Also if you are a bit more adventurous, there is the whole North-East USA including Washington DC, New York and Boston. I have done the rounds of the above (starting from Toronto) on a road trip myself. You could cover a large enough area within the timeframe you mention.