You don't mention whether you prefer quick and cheap, vs a nice sit-down meal.
For quick and cheap, there's always
Pret a Manger, for freshly-made sandwiches and salads. They have a location on Monroe that may be a bit closer to the Federal Buildings, whereas the one on LaSalle may be closer to the Sears Tower. There's also the
French Market, not too far from the Sears Tower, with all of its various vendors (including
Saigon Sisters for Asian,
Pastoral for cheese and sandwiches, and
Vanille and
Delightful for pastries).
For a nice sit-down meal, two contemporary Italian places are near the Federal Buildings -
Vivere in the Italian Village complex, and
The Florentine in the JW Marriott. And for contemporary American,
Blackbird is one of our best, not too far from the Sears Tower and offering their $22 three-course prix fixe lunch menu.