You should have a relaxed visit with that much time. I wouldn't rent a car, too much to do downtown and distances are too far to anything worthwhile in the suburbs for a first-time visitor, unless you are really interested in botanical gardens or something(and even then you can get there on a bus/train). And that's coming from a suburbanite. Get a 7-day cta pass and just enjoy the city itself. Make sure you have some deep dish pizza and a good italian beef, make sure you visit a blues bar some night(a real one, not "House of Blues", sorry), of course do the architecture cruise as was mentioned, walk Michigan from Oak south to the Art Institute or a touch farther, and to echo the other PP, the Hopper exhibit should be worthwhile also... if you care, check out Chinatown, Navy Pier, Greektown, and of course check out "Chicago must do's" on tripadvisor for more ideas. Oh ya there are double decker bus tours that are decent for getting an overview/lay of the land.