The 96th floor of the Hancock Building has a lounge. There is no charge to get in, and although cocktails are $9-10, the view is unbelievable.
The subway ride will cost $1.50 each way and I'd recommend it. You can plan your trip at
http://www.yourcta.com -- click on train maps, and you'll see the Blue Line from ORD to downtown and the Red Line to State and Chicago, from where it is a four-block walk to the Hancock Building.
Enjoy your brief stay in Chicago and good luck with your travels!