If your only "must do" is the Art Institute, depending upon what you are interested in seeing, you can easily spend the entire day there.

Everyone talks about the Architecture Foundation boat tour, but there are many other tours that are cheaper and probably just about as interesting, although not as beautiful. The Architecture Foundation office and shop are almost right across the street from the Art Insitute in the Santa Fe Building. I believe they offer the historic architecture (Chicago School, Beaux-Arts and Art Deco) and modern architecture tours several times per week.

There are plenty of fast food alternatives within a block or two of both of these places, as well as a cafeteria in the AI.

A block away is also Millenium Park, which is quickly becoming the most popular tourist attraction in the city. An architecture fan is not going to want to miss the Frank Gehry band shell and bridge, among other things. Take your sandwich there and enjoy one of the greatest new downtown attractions in any city.