You can grab a coffee or a juice right next door to the KINDL Center in the old brewing Hall at König Otto, complete with all the fascinating brewing equipment from its completion in 1930 it is a hall reminiscent of German Expressionism style… or as stated on their website. :)
Wim Wenders shot Wings of Desire here. It's a quiet place to work and definitely a gorgeous place to visit. Combine it with a visit to the Philharmonic, just across the street. Affiliation is 30 euros per year and you have lots of desks all over the library to choose from.
While strolling around Mitte, check out the flagship store of Berlin´s cult women fashion brand. (And try a delicious cinnamon bun at Zeit für Brot next door.)