Would agree having a drink in the Signature Lounge of the Hancock - no need to pay for a visit to the sky deck - this is better !
Heaven on Seven - I usually make a reseravation to be sure for weekends
Walk along Lake Shore Drive .. for as far as you want - having a quick look at Millenium Park on the way.
would say that the Art Institute is well worth a visit
Catch a show whilst in town - Blue Man Group ?
trip to Lincoln Park (also the zoo is there which is free - 151 bus will take you there)