Two other restaurants close to the AI:
1. Russian Tea Time is a reasonably priced restaurant that features Russian food.
2. The Berghoff is a Chicago institution featuring German food (and locally-brewed beer and rootbeer) although it has lost some of the luster from its heyday. There is both a sit-down restaurant and, in the basement, a less-expensive cafeteria-style "cafe," with a stripped down menu. I still enjoy their Viener Schnitzel and the creamed spinach, which are available both in the restaurant and cafe.
(As to Terzo Piano, I think that there food is pretty good for what it is but the menu is fairly limited.)
Berghoff "closed" with big hoopla about 8 years ago, supposedly to convert the space to catered private parties. About 6 months later they reopened a portion of the space as a restaurant under some other name (derived from the street address). At some point, about 6 years ago, the entirety of the space was once again open as a restaurant and they resumed using the Berghoff name. I eat there for lunch every few months, most recently about 1 month ago. Definitely still open.
nxasty listed most of them- Out of the nicer sit-down places I would recommend tesori or The Gage/Henri. Park Grill is too touristy, never found anything special at Miller's Pub. I also like Terzo Piano, it has good food and a great atmosphere.
I work in the area and often lunch here- for something simpler, I would second Pastoral or Hannah's Bretzel for great sandwiches. Max's has some of the best hot dogs in the city. For something a little different, try Native Foods for flavorful and filling vegan food. Although it's a bit of a walk, I also love Benjyehuda for falafel and great fries.
Actually, Terzo Piano, the restaurant inside the new Modern Wing of the AI from Tony Mantuano of Spiaggia, is very good indeed. It's priced comparably to other Loop places for similar menu items. Typical pricing in the Loop for lunch at nice sit-down places is sandwiches $10-13 and entrees $15-20. Terzo Piano is consistent with that, except that they don't have any sandwiches.
Other good places nearby include:
tesori (Italian) - www.tesorichicago.com 18
Park Grill (American) - www.parkgrillchicago.com
The Gage (gastropub) - www.thegagechicago.com
Henri (French) - www.henrichicago.com
Miller's Pub (American) - www.millerspub.com
Heaven on Seven, Wabash (Cajun/creole) - www.heavenonseven.com
You'll probably spend less at Miller's Pub or Heaven on Seven than you will at Terzo Piano or tesori because Miller's and Ho7 both have an emphasis on sandwiches on their menus.
Another option is Pizano's on Madison, for deep-dish pizza - www.pizanoschicago.com - which will be less expensive but only if you have someone to share your deep-dish with.
The above suggestions are all for nicer restaurants where you are seated and waitstaff take(s) your order, and most of them accept reservations, thereby avoiding long waits to be seated.
As another option which is much less expensive, you'll typically spend a lot less at places where you wait in line to place an order, like at a fast-food restaurant, and some of these are a definite step above the nationwide burger chains. In particular, there's a nice selection of freshly-made sandwiches and salads at Pret a Manger (nearest location to the Art Institute is 108 S Michigan, right across the street), Pastoral (53 E Lake), Hannah's Bretzel (131 S Dearborn), Au Bon Pain (122 S Michigan), etc., all of which have multiple locations downtown. All of these places have limited seating (for which you may have to wait) and do a big carry-out business. There are typically lines during the weekday lunch rush but they generally move quickly. If it's a nice day, you may wish to take your lunch over to the park to eat it. Also note that some of these places may be closed weekends and holidays.
See this discussion for more ideas:
Lunching in the Loop - www.chow.com/topics/527275