Breakfast in downtown chicago
Where will you be staying (or spending time)? "Downtown Chicago" is often used to refer to quite a large area, roughly 3 miles from the South Loop to the Gold Coast.
You can get some ideas for breakfast and brunch places from the discussion at www.chowhound.com/topics/364403 (the recommendations there are still current). Note that the selection of places that do Sunday brunch is vastly broader than those that are open 7 days for breakfast.