what do you like to do? touristy stuff? off the beaten path stuff? nerdy stuff?
I'm not a big fan of the touristy stuff myself. I lived in Chicago for a year + 1/2 and never went to Navy Pier.
The Chicago Architecture Foundation (
http://www.architecture.org/) does some terrific walking and driving tours of buildings throughout the downtonw and the city. highly reccomended.
The Sears Tower (or whatever it's called these days) is worth going up to the observation deck --- it's like $8 but you can see the entire city. highly reccomended.
The John Hancock (there's a bar on one of the top floors) is another great way to see the city from above.