It depends on how late your flight from Chicago is delayed I haven't had too much luck with flights in the US the last couple of times so that 6 hours may turn out to be less. Also it depends on where the next flight takes you.

Otherwise yes, you should be able to get out again depending on the day and time. If you hit rush hour, the metro would be the best choice. If not then a taxi might be the quickest use of your time. They are not too costly from the airport to the major areas.