I hope I'm not too late to the party. The Art Institute is a top-notch museum. Take a lunch break by walking straight out of the main entrance, crossing Michigan Ave and continue down the left side of Adams Street. Less than 1/2 a block down is the Russian Tea Time Restaurant. Have afternoon tea before going back for more art.
There are great views of the city from the Hancock Tower. Instead of paying for the observation deck, go to the bar on the 95th floor. They usually let people in to have a look at the views even without getting a drink (though that's not a bad idea either). Whether you go in the bar or not, there are excellent views toward the Loop from the free bathrooms. \
Oak Park is a great day trip. Take the Lake Street el. Then walk to the Wright Studio for a tour. You can also get a self-guided alking tour of the area - a good number of early Wright homes and contemporaries - interesting to compare. If you go to Robie House, be sure to allow a couple hours to walk across the street and wander around the University of Chicago campus. If you only have time for one, I'd choose Oak Park. After your walk, stop at Oberweis, 124 N. Lake St., for some of the best ice cream in Chicagoland.