Although ‘only’ 94 floors up, Hancock tower would be my personal recommendation as you are not limited by time as opposed to the Skydeck at Sears tower which gives you a 30 minute window before you have to come back down. This isn’t the case for Hancock, you can spend as much time as you like up there, enjoy 360 views of the city and lake as well as grab a drink from the café, or just one floor away there is the Signature Room which is a bar/restaurant where you can dine amongst the clouds. Also at the Hancock tower you can go on ‘The Tilt’. 94 floors up the glass window tilts down towards the ground whilst you stand against it.
"Sears" tower (Willis technically) is the tallest tower in Chicago, it has great views of the city and you can even visit the Ledge which is the glass box in which you can stand inside on the 103rd floor. It requires you to buy your ticket in advance, may only have to be 24 hours but will need to plan ahead.
In my opinion the best views of the city skyline can be seen from the planetarium. Definitely worth a visit, even if just to enjoy the view.
Taking a walk down Michigan Ave. is one of the things you have to do when in Chicago. Starting at the Art Institute and start walking north up Michigan Ave. You will quickly come across The Bean, Millennium Park, and Maggie Daley park all which are worth taking some time to explore. Then you can take the walk up towards the Hancock tower, walking over the river.
Located on the walk highlighted above along Michigan Avenue, this is an amazing art museum with artwork and exhibitions from every style and era that you can imagine. You can easily spend all day wandering around here.