Delta flies to Chicago from all the hubs (JFK, Atlanta, Detroit and Minneapolis). It is called the Windy city and it lives up to it. In September was way colder than in Montreal. I got very bad weather. Chose a hotel close to the airport which means I was really far from the city. So associating bad weather, long distances and short trip I end up not exploring much.
Renting a car in Chicago is a very bad idea, it is very hard to park, and the payed parking are very expensive.
Eating the pizza (deep dish pizza) is a must.
The architectural tours is interesting as well as going up the Sears tour. But honestly, I am not looking forward to go back to Chicago.
