Not having to dress up too much isn't a problem. You can get by with "business casual" (non-blue-jeans slacks, shirt with collar, etc) just about anywhere except fewer than a dozen of the fanciest, most expensive places.
You might want to avoid
Avec at prime times, like Saturday night, due to their no-reservations policy. (And I don't know about your parents, but mine wouldn't really appreciate the communal seating, either.) I would recommend
Blackbird, next door and co-owned, but I just called them last night and they didn't have anything open for Saturday night between 5:00 and 9:30. Wherever you decide for Saturday night, you should really make a reservation.
For a nice meal, I'm particularly fond of
North Pond. The food is wonderful (contemporary American, fresh and local ingredients where possible) and the setting in the middle of Lincoln Park (the park, not just the neighborhood), at the end of North Pond with the skyline visible at the opposite end of the pond, is exquisite, day or night. Also open for Sunday brunch.
There are lots of other great places for contemporary American food in and near downtown -
one sixtyblue,
Aigre Doux,
Custom House,
Naha,
MK,
Sweets and Savories,
Spring, etc. In addition to these, other excellent mid-priced places in the greater downtown area include
Cafe Spiaggia,
Coco Pazzo,
Vivere, and
Trattoria No. 10 for Italian,
Bistro 110 for French bistro food,
Shaw's,
Hugo's, and
Fulton's for seafood, and a whole bunch of steakhouses. My favorite of all of these, aside from North Pond, where I just went last weekend, is one sixtyblue; as of last night, they had plenty of availability on opentable.com for this Saturday night. Many of our restaurants accept reservations on opentable.com and it's a handy way to see at a glance which ones have availability at the times you're looking for.
For another meal, but less expensive, I would take them to a place for authentic Chicago-style deep-dish pizza. Take your pick - double-crust "stuffed" pizza from Giordano's, Edwardo's, or Bacino's, or single-crust "pan" pizza from Lou Malnati's, Pizano's, Gino's East, or the original location of Uno's - you can't go wrong with any of these. You can phone your order ahead of time if you want to avoid waiting 30-45 minutes while seated for your pizza to bake; you can find their menus on their websites.