Hello, all!
I will (with 95% certainty) be in Chicago for 5-6 Days in October this year. Those of you from the US, especially from Illinois or even better Chicago itself: Does anyone of you have good or especially "secret" tips on what to do? Also the general stuff is interesting 
My dad fancies to see some of the (old) butcheries that Bertold Brecht wrote about and both of us would like to follow some of those sites of famous robberies and things like that by Al Capone and co.
Thanks already for input and ideas 
Jona L.
In all seriousness, I haven't been to Chicago in 15 years and there is a reason for it. I'd say a 3 day visit is the maximum I'd suggest because as mentioned, there simply isn't enough to do there to fill up 5-6 days. Personally, I've been to ~40 out of the 50 states, have lived in 5 different ones, and I grew up in the city (Cincinnati). The only things to do in Chicago are "man-made" versus being able to go do something in nature. Heck, even the Chicago River is man-made since they reversed the flow. If you are renting a car, I suggest spending a few of those days in Door County, Wisconsin (~5 hours from Chicago) or the opposite shore of Lake Michigan (~3 hours). Door County is where all the people from Chicago vacation and they are commonly known by Cheeseheads as FIB's (F*cking Illinois Bastards) Since you're going to Chicago, here is my list of things to do:
1. Wrigley Field -- Since you're visiting in October, the Cubs will already be out of playoff contention and tickets will be cheap and plentiful.
2. Sunset River/Harbor Cruise -- Probably not as scenic as it used to be now that former Mayor Daley destroyed Meigs Field.
3. Navy Pier/Boardwalk -- The boardwalk isn't populated with stores like other big city boardwalks, but is a nice trail to walk/run/rollerblade/bike along the shore.
4. Moto Restaurant -- You mentioned you were going to the meat packing district and Moto is one of the more cutting edge restaurants out there in the world of molecular gastronomy:
http://www.motorestaurant.com/5. Ed Debevic''s Restaurant -- One of the restaurants where the servers are rude and treat you like crap on purpose. Entertaining nonetheless:
www.eddebevics.com/6. Midieval Times -- If you ever saw the movie "The Cable Guy" with Jim Carey you've seen this before. Since your not in the states, this will probably be a novelty for you. It's the only place (they're several actually) where you can watch live jousting while eating a whole chicken with your hands and chasing it with a goblet of Coke:
http://www.medievaltimes.com/7. Sears Tower (now called Willis Tower) -- I went 15 years ago when it was the tallest building in the world still. The elevator ride up was the highlight as it is an express elevator going 100 stories up in a minute or so--if you jump up and down on the way up/down it's really cool. Seeing Chicago from the top wasn't anything over-the-top or spectacular--if you've seen Chicago from an airplane it's the same thing.
8. Chicago Art Museum -- Art museums bore the hell out of me, but my mom was a member for so many years when we flew for free on American/Delta. She'd fly up for the day to see exhibitions and it's one of the top art museums in the nation/world.
Today, Chicago is one of the hardest hit cities in America economically. The corruption of the unions and political leaders has turned it into Detroit Jr. Chicago in recent years has been known for students getting gunned down at record rates and there are statistics out there showing that you're more likely to get shot and killed as an American in Chicago than in Iraq.