There are direct flights from ORD to all four of the cities mentioned and you would very easily fill 3 days no matter which one you choose.
Quebec: 400th anniversary. Lots of things going on. The only walled city in North America.
Montreal: One of the best cities in the world.
Ottawa: Filled with green spaces, museums, restaurants.
Toronto: A world class city (just ask anyone who lives there

). Only 1.5 hrs to Niagara Falls. Shortest flight from ORD.
My pick: Quebec City. I'll be there for three days this weekend.
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13F