Millennium Park (The Bean) and Grant Park (Buckingham Fountain) are free. There's also the Lurie Garden there. The riverwalk is also free. Just walking on Michigan Avenue is great, or shopping on State Street. There's a great statue of Lincoln at Michigan and Wacker (across from the Wrigley Building and next to the Tribune Tower). WGN radio broadcasts to the sidewalk from the Tribune Tower -- you can watch the radio show from the sidewalk. Lincoln Park Zoo is free.
Navy Pier has got good restaurants and seasonal stuff -- they've got some great fireworks Wednesdays and Saturdays in July.
The Art Institute (near Millennium and Grant Parks) is amazing--if you're into art (there's also stuff by Frank Lloyd Wright and some cool miniature rooms. It's not free, though -- I think it's like $25/pp. The Field Museum, Adler Planetarium, Museum of Science and Industry (there's a submarine) and Shedd Aquarium are also great if there's interest in your group.
The Willis Tower (formerly Sears Tower) has a skydeck -- I've not been, but if you're looking for a view.... Also, the former John Hancock building has a great view.
You'll want to get some Chicago-style pizza -- Uno's or Giordano's.
You'll definitely need to pick and choose -- there's way too much stuff to try to see in a weekend. Have a great time!