Where will you be in "downtown Chicago"? The neighborhoods comprising downtown are spread out (3 miles from north to south) and surely you'll want something conveniently close to where you'll be.
Also, what kind of lunch place are you looking for? A place to get something to go, to eat in the park? An inexpensive spot for fast sandwiches or burgers? A nicer place for a relaxed sit-down meal?
Okay, it sounds like you're interested in a nicer place for a relaxed sit-down meal. Where will you be in downtown Chicago? In River North near Wildfire, or along the Magnificent Mile, or in the Loop, or the West Loop, or the Field Museum and South Loop, or around the Art Institute and Millennium Park, or...
If you're around the north end of the Mag Mile, I'd go to Cafe Spiaggia for Italian.
If you're around the south end of the Mag Mile, I'd go to the Purple Pig, for charcuterie, pork, etc.
If you're in River North and you're able to eat on the early side (say, get there before 11:30), I'd go to Frontera Grill, for creative Mexican food; other good places for lunch in River North include Cafe Iberico for tapas, Sable for contemporary American small plates, and David Burke's Primehouse for steaks and burgers (which they call "burkers").
If you're in the Loop, Atwood Cafe is a good stop for contemporary American, and the Florentine or Vivere for Italian.
In the vicinity of Millennium Park and the Art Institute, Park Grill has American food including good burgers.
If you're in the West Loop, Blackbird is good for contemporary American.
In the South Loop, Mercat a la Planxa for tapas, Custom House Tavern for American, or Hackney's for burgers and an onion "brick".
These are all nice sit-down places. (Most accept reservations, but it sounds like you're going to be playing things by ear.)
619 W. Randolph, Chicago, IL 60606
737 N Lasalle Drive, Chicago, IL 60654
David Burke's Primehouse
616 N Rush Street, Chicago, IL 60611
445 N Clark St, Chicago, IL 60654
Hackney's Printers' Row
733 S Dearborn St, Chicago, IL 60605
980 North Michigan Ave, Chicago, IL 60611
Mercat a la Planxa
638 South Michigan Avenue, Chicago, IL 60605
Custom House Tavern
500 S Dearborn, Chicago, IL 60605
1 West Washington St., Chicago, IL 60602
11 North Michigan Ave, Chicago, IL 60602
71 W Monroe Street, Chicago, IL 60603
Sable Kitchen & Bar
505 North State Street, Chicago, IL 60654
151 W. Adams, Chicago, IL 60601
500 N Michigan Ave, Chicago, IL 60611