Like everyone else, I have to say hit the museums. I like the Art Institute, which has a few of the “Hey, I know that painting!” paintings.
Sears Towers is a typical tourist thing. Might as well since everyone will ask you about it back at home later.
If you’re the shopping type, Michigan Ave and the Loop in general has all sorts of fun stores. If you want… umm… odder stores, try up on North Clark where they hide all the fetish/new age/etc type stores.
Back to museums, the new Museum Campus is a day trip worth doing. Field Museum of Natural History, Shedd Aquarium & Adler Planetarium all in one spot (more or less) and the Adler’s just re-opened, so they have neat stuff going on.
Hit the lake. It’s mighty cold water, but a nice thing to walk along.
Really, without sounding like a commerical, there’s tons of worthwhile things to do in Chicago and it’s hard to waste a day if you at least leave the hotel.
“I guess it is possible for one person to make a difference, although most of the time they probably shouldn’t.”