I'm not really a fan of Russian Tea Time, I found it to be, well, too Russian in the wrong way or sense, as in surly service, though their cocktails are reasonably priced for after work (though not as good as Ceres in the CBOT building). I prefer the restaurant next door - no, not Panda Express, on the other side, Tesori. Just a block west on Adams is BeinMeSabe, a Venezuelan/Colombian Arepas (dumpling/empanadas) which is really good, though I don't know their hours anymore.
If you are looking for something unique, Italian Village on Monroe is worth a visit (no elevator to the basement or second floor, which are quirkier than the expensive restaurant on the main floor, however) - ok food, but one of the oldest restaurants in Chicago and retro decor.
In the 300 S. Riverside building (immediately above the south platforms) there is a lunch/cocktail place called Little Toasted which isn't bad and has outdoor seating for good weather (Chicago has been cold and foggy so far this "summer").
Pizano's is definitely better than Giordano's. However, near Union I recommend Beggers Pizza, a south side chain, at 310 S. Clinton which is literally kitty corner across the street from the SW corner of the Great Hall. And is pretty darn good!
Some office buildings, such as the Board of Trade (in the basement) still have cafeteria's rather than "food halls" (gussied up food courts to my mind) which can be pretty good. Most places in the loop are now pricy fast casual lunch places these days rather than old school lunch grills, though there are still a few. There's a Chicken Planet at Wells and Van Buren which does decent roasted chicken with sides - not fancy, but I think they have a couple of tables. Greek Kitchen at Wells & Van Buren is ok too.
Manny's at Jefferson and Roosevelt is pretty unique and inexpensive - you might also spot local political celebrities there (though you probably don't know then on sight).