I'll add a few things to what lee has already mentioned...If the weather is nice, you absolutely have to go on the Wendella Architectural Boat Tour...It's fantastic. As for pizza: I'd recommend Pizzeria Due, Pizzeria Uno, or Gino's East (all downtown in the Streeterville neighborhood). If you like jazz, check out the legendary Green Mill. In addition to the Field Museum, which is great, the Museum of Science and Industry is also a lot of fun. Skip Adler Planetarium, though, which, in my experience, is the weakest of the major Chicago "museums" (using that word loosely).
If you get a chance, take in a show in the vibrant theater district (it's not New York, but it's still got a lot of great shows). Personally, I'm a fan of the Goodman. There are also a lot of great smaller companies--check the Chicago Reader for reviews.
Also definitely go see the Chicago Symphony Orchestra (tickets are often pretty cheap, esp. if you're a student) and, if possible, the Lyric Opera. Both well worth it. And the CSO often has non-orchestral things; I saw Ravi Shankar (and even met him) there last year, for instance.
As far as eats besides pizza...Amazing Mediterranean/Middle Eastern food in Wicker Park, and also a lot of different really good restaurants in Andersonville. Perhaps the best Indian food in America in Little India on Devon--it's legendary amongst Indian Americans.
If you're into NPR, "Wait, Wait Don't Tell Me" records live every Thursday and tickets cost about $20.
That's all for now, I'll post again if I think of anything else.