This exquisite, beaux-arts building began its life as the Chicago Public Library in 1897. Today the block-long building houses terrific art exhibitions (especially the 4th-floor Yates Gallery), as well as jazz and classical concerts at lunchtime (12:15pm most Mondays and every Wednesday). It also contains the world's largest Tiffany stained-glass dome, on the 3rd floor where the library circulation desk used to be. InstaGreeter tours of the Loop depart from the Randolph St lobby, as do Millennium Park tours. And it's all free!
Oh, and there's more. Free foreign films screen on Wednesday at 6:30pm from June through September. StoryCorps’ recording studio (where folks tell their tale, get a CD of it and have it preserved in the Library of Congress) operates on Thursday (noon to 6pm) and Saturday (10am to 4pm). Check the daily schedule posted at the entrances (at both Randolph and Washington Sts) to see what else is going on.
The Gilded Age interior mixes white Carrara and green Connemara marble throughout. The building's splendor was meant to inspire the rabble toward loftier goals. You can explore on your own, or take a free building tour (1:15pm Wednesday through Saturday), which departs from the Randolph St lobby. There's also free wi-fi and seating areas throughout the building.