What is the number one thing to do if you're just visiting Chicago for a day?

If I had only one day in the Spring, Summer or Fall, my number one thing is an architecture cruise on the Chicago River. If I had only one day in the Winter, I'd skip the Willis/Sears tower and make my number one thing a visit North Michigan Avenue. If you walk from the river north to the John Hancock Tower, you'll enjoy one of Chicago's signature experiences. In my opinion, the John Hancock tower is a more beautiful example of modern skyscrapers than the Sears tower. And you get better views of the lake from the observation deck. But if you want to look 1000 feet straight down from the top of a skyscrpater, then you have to go to the Willis/Sears Tower SkyDeck.

Drive down Lake Shore Drive and head to the Adler Planetarium. From there you will get the best skyline photos.

I would go to Navy Pier, and the Willis Tower then a cruise on the River!

It all depends on your personality and what you like to do when you travel in general. With Chicago being the 3rd largest city in the U.S. trying to single out just one thing that would suit everybody would probably be pretty tricky.

Give us an idea of what you like to do and that'll help us guide you toward some great Chicago sights / sounds / tastes!

In the warmer months, I am a huge fan of the Chicago Architecture Foundation river cruise. I've lived in Chicago most of my life, but I enjoyed the cruise both times I did it.

http://www.architecture.org/page.aspx?pid=574

If it's around the holidays, a visit to Macy's (formerly Marshall Field's) and having lunch at the historic Walnut Room is a must. Be prepared for long waits on weekends between Thanksgiving and Christmas.

http://macysrestaurants.com/walnut-room-2/

Impossible to answer....depends on the season, the people/ages, and weather (-3 today!) I would use this site to find the appropriate venue/attraction to see. http://www.tripadvisor.com/Attractions-g35805-Activities-Chicago_Illinois.html The bus tours are a good way to see a lot of the city. http://www.grayline.com/things-to-do/united-states/chicago/.

I'd visit Willis Tower, which used to be called the Sears Tower, the tallest building in the world for years, then head over to Navy Pier if it's a nice day. The Ferris Wheel is beautiful at night.