Chicago is one of the world's great cities, with great dining, culture, architecture, museums, and shopping. You could spend a week and only scratch the surface, so it's a good idea to do some research before spending your 48 hours.
The links from mamaprincess are very good. Also add "Choose Chicago" at
http://www.choosechicago.com/ to the list. It's the official Chicago Convention and Tourism Bureau website.
Unless you're very lucky, Chicago will be quite cold between now and the end of March.
In addition to excellent (and expensive) gourmet food, Chicago is also known for terrific "blue collar" food, such as Chicago-style pizza (my favorite is Gino's East), Chicago hot dogs (Vienna beef with all sorts of toppings), and Italian beef sandwiches.
The Chicago Art Institute is one of the world's great art museums.
As a live theater town, Chicago is only behind New York and Los Angeles in the United States. If you plan ahead, you can get tickets to everything from big Broadway musicals in magnificent theaters to cutting edge dramas and comedies by theater companies such Steppenwolf and Goodman. If you prefer the flexibility of waiting until the last minute, you can buy half-price (plus a service charge) tickets through Hot Tix at
http://www.hottix.org/, but your selection will be limited to shows that aren't selling out at full price.