This University of Chicago library cafe provides a good break from the rigors of the Regenstein Library (it's located on the A-level). "Unofficial product list" includes a laundry list of candies and sweets; Gatorade, Snapple and drinks in that genre; assorted bagels; and Clif and Luna bars. There is also a variety of sandwiches/Thai/sushi/pizza delivered by places like Briazz, Kikuya, Nile, Thai 55 and Food Evolution. The regular coffee (all you can get) is a thick, all-nighter type brew.
Average cost: <$10