We lived downtown for years and never needed a car. Lived in Sandburg Villiage and caught the bus at the Ambassador hotel to the airport. Piece of cake.
Perhaps you ought to make a trip up to ORD and check out ops for flyers and postings for roommate and crash pads. I was there on sit time last month and saw a lot of stuff on the bulletin boards. Spoke with one girl who just stays in ops. They have WiFi now and she has a membership at the Hilton's Fitness Center, so she can excercise and shower there before going to work.