Late to the party, but my two sense from my time living there (albeit, too briefly):
- Cubs game in the bleachers. That goes without saying. General admission for a large part of it so get there early to sit close to the wall. You might be lucky enough to hear Ron Santo yell out "This is the Year!!!" for yet another year.
- Pizza. Take your cholesterol reducing meds before heading into any authentic joint. My personal favorites: (1) "The Art of Pizza" on Ashland just a few blocks north of Diversy. Voted best Chicago pizza back-to-back years in 2005-06 I believe; (2) Ginos East at the corner of Lincoln and Diversy (you can tell where I lived).
- Architecture - take the boat tour of the Chicago River. The different architecture of the Chicago skyline is amazing even for those they may be skeptical. Chicago really was the birthplace of the modern skyscraper. Also, the two most recent Batmans were shot in Chicago so there's lots of recognizable scenes (including all the draw bridges across the Chicago River.
- Grant Park. Really nice park and a lot of cool cafes that border on S. Michigan Ave. Check out the Art Museum near Grant Park too - very cool and free on Tuesdays (IIRC). Has a cool little outdoor restaurant and bar that's near the foundatins - worth grabbing a cold one if it's hot/humid out. And of course, "The Bean." Be one of the thousands to take a picture of your reflection in the giant chrome bean.
- Michigan Ave aka Magnificant Mile. The heart of Chicago shopping.
- The Loop - the elevated train tracks in downtown form a loop. If you are looking to kill time down town, this is your best bet for a perimeter as to find places to eat and to not wander too far south (you have a few blocks south of the Sears tower before things get a little dicey).
- Lincoln Park - check out the Lincoln Park "Trizzies." Basically sorority chicks from Big 10 schools manage to find their way here en masse. Good for another type of sightseeing.

Additionally awesome food/bars - stick to Clarke Street.
- Wrigleyville. Obviously the area that surrounds Wrigley. The Cubbie Bear is the most famous of the pubs, but I would stick to the smaller ones as the Cubbie Bear sells out frequently to the hip-hop crowd or techno music immediately after Cubs games (even in the middle of the afternoon - weird). Shefflield, Addison and Waveland (behind the bleachers) are your ticket streets to hit and Clarke has a ton of bars north and south of addison.
- Sears Tower/John Hancock Building. The two "anchors" to the Chicago skyline. I prefer the Hancock, but the Sears (formally known as pretty soon, IIRC) is more famous. Apparently, on a clear day, from the observation deck of the J.H., you can see 5 states - Illinois, Wisconsin, Michigan, Indiana and Ohio. Additionally, just some great views of the city.
- Bike ride - if you are an activity type - rent a bike and roll along Lake Shore Dr. from Lincoln Park to Grant Park.
- Navy Pier - pretty cool. Definitely wasn't the best part of the city, but if you need to kill time waiting for shoppers, it's worth the walk, just to get out on the lake a little and look back on the skyline.
I live in San Francisco now and if it wasn't for the weather, I would definitely consider heading back to Chicago. I
really like that city. PM me for more details if you want!