I'd say that if you took the train downtown you might have as much as 1 hour to explore. RT will cost $4.50. You could take the train to wicker park and have maybe 1 hour and 15 minutes to explore that neighborhood. Not that much else I can think of within easy access of the blue line train.
IMO, even if the only thing downtown you have time to see is Millenium park it is still worth it. The park is free and the train will be $4.50. Better than spending 4 hours at O'hare.