you'd probably be better to go to the top of the John Hancock tower. The bar at the top (It used to be called "the 95th", it may still be) is a bit pricey, but you can't beat the view!

And since you're there, right across the street is: 1) the historic water tower, 2) The Chicago Store, which has infor on Chi-town and souvenirs, etc. and 3) Water Tower Place, a 10 story mall that contains the aforementioned Marshal Fields dept. store. (Billy Goat's is just down the street, too!)