One of my favorite things are the boat tours on the River/Lake, if you really are an architecture geek, go for the CAF River Tour (I see tickets on Hot tix for 1/2 price a lot) or if just wanting a nice boat ride with commentary, do the Wendella or Shoreline.  The Chicago Architecture Foundation also has a variety of walking tours.

The Chicago History Museum is a great place to learn about the history of Chicago.

The Chicago Cultural Center has a tourist office and they also have instagreeter tours in addition to the regular Chicago Greeter program.

Renting bikes to ride along the lakefront might be something you'd enjoy, I haven't done a tour with Bobby's Bike Hike for many years but you might find that enjoyable or you might just want to ride by yourselves.  The Lakefront path is about 18 miles, more congested going north than south.

The Chicago Jazz Festival is Labor Day weekend.

All of the Museums are great, depending on your interests, the The Art Institute of Chicago, Museum of Science & Industry (although quite a way from downtown and perhaps not feasible for a short visit), the Shedd Aquarium, a little known secret is that if you just want to see the main part of the aquarium, you can get a ticket just for that, I think it was $8 the last time I did it.  The Field Museum has an exhibition on the 1893 Columbian Exposition that I haven't gotten to yet.  There's also the Adler Planetarium, Museum of Contemporary Art and a bunch of smaller museums.

I'm a little out of your age range! But here are a few ideas:

• The Chicago Jazz Festival and the North Coast Music Fest (probably more your speed) are both that weekend in downtown Chicago 
• Although it can be a little crazy and younger (but you'll fit right in!), going to Castaways (a beach bar) and having a beach day would be fun with good weather
• If you're not totally into museums, we like the Chicago Architectural Boat Tour - you get to be outside in good weather, on the water, and still learn something cool about Chicago's history. Plus, they have a bar :) You could also try to rent a boat and have someone drive you around for the day on Lake Michigan 
• For drinks- in the summer time I recommend something with a view. West Town/Wicker Park/Bucktown or Wrigleyville may be a good area for you for bars, but if you're looking for something different with a view try the top of the Hotel Lincoln - A Joie de Vivre Hotel - trendy, fun and great scenery 

Hi Kristen, you're going to want to go to Wrigleyville for nightlife, there are even a few bars open to 4am I believe.  You should definitely check out Millenium Park to see the bean, lay in the outdoor auditorium grass, and see the fountains.  For art, go the art Institute of Chicago

My favorite cultural or Chicago-specific things in the city are:

• Architectural Tours (There are many Frank Lloyd Wright buildings in Chicago)
• The The Art Institute of Chicago
• Museum of Science and Industry - this can be an entire day in itself
  
• Rent bikes in Millenium Park and ride along the Lake Front Path
• Many people also like to visit the  Hancock Building or walk down Navy Pier, but it seems pretty class tourist to me

For more food & fun, some places to consider are: 

• Go to brunch, almost anywhere, and ranges from casual to extravagant. 
• Get  lunch at Big Star for tacos and margaritas, where you can sit outside ...
• ... then savor classy cocktails at Violet Hour (there may be a line), which is a speak-easy style bar
• Hubbard Street has several bars, where you can bar hop from one to the next and end your night at The Hangge-Uppe

I will be going to Chicago for my third trip also on labour day weekend. The first time I went, I visited the Adler Planetarium which is also near the The Field Museum and the Shedd Aquarium Store. This allowed us to walk along the lake and see the Buckingham Fountain and the Versailles modeled gardens in Grant Park south of the river. We might rent bikes this time to see more and farther. Last time we went to the Lincoln Park Zoo which was a lot of fun, even as adults...and it's free. Take a boat ride. We took one that included going on the lake and the river which discussed a lot about the history of the city and the architecture. Great photo opp too. Have fun!