The Willis Tower is the tallest building in the world and the landmark of Chicago after it was built that year. The queue time may be longer, but if it's rainy, there may be fewer people. If you look at the whole Chicago from the Willis Tower, you can take your own photos. If you don't lack money, you can use the camera of the viewing platform and finally wash out the photos as a souvenir. The price is 30 dollars a piece.