What time of day and how much are you looking to spend per person?
Sears Tower is the Loop, so there are tons of restaurants catering to business diners, so you have lots of options. I would recommend that you go here:
and punch in Loop and Grant Park as your neighborhoods.
If you're looking for true Chicago cuisine, I'd recommend the Berghoff on Adams Street -- the food is only OK, but the restaurant is a Chicago institution (for years, they had the first liquor license issued after prohibiton ended, although they closed for a while a few years ago and may have had to give up the coveted Liquor License #1).