Since you'll be downtown:
- Chicago Hot Dogs: Gold Coast Dogs - 159 N Wabash Ave (or at Union Station)--get the cheese fries
- Comic Books: Graham Crackers - 77 E. Madison St.
- Pizza: The Art of Pizza has about the best, but that's way up north (Ashland and Belmont). Forget pizza, you can get stuffed pizzas all around now. But Uno, Gino's and Giordano's are the big names for stuffed / deep dish pizza.
- Record Store: Reckless has a downtown location - 26 E. Madison (near Graham Crackers). Dusty Groove is good, but at Ashland and Division.
If your extra two days happen to be BEFORE Pitchfork, The Art Institute has free evenings on Thursdays. (was just there last Thurs.) I couldn't find any other free days during that time-span. All the museums used to have free days each week, but no more!
And stare at the Picasso for a bit (50 W. Washington St.):

(I used to work in the building across the street from the building behind this).
And just behind him, across the street to the left is this crazy/cool building:


And of course, the obscenely over-priced (EDIT: cost to the city--free admission), most massively delayed (yet absolutely stunning) project ever, Millennium Park:

(EDIT 2: Quite the massive pic, requires left-rightscrolling)
That's about all I can think of right now.