Things to Do In Chicago

Out of sight and out of mind. That's how I feel about Chicago sometimes. Not really a good thing as quite a few people are asking me for recommendations on what to do when on a possible visit to the Windy City. So for those of you sitll there, I'm looking for recommendations. Tell me what you would consider a must see/do for someone who is visiting for the first time. In the mean time, I have got to put together my own list of favorite places (restaurants, bars, Museums, etc.) before it all becomes a blur.