The Ballhaus ist a brand-new hostel in Mitte right at the U-Bahn station Naturkundemuseum. The area is quite expensive in general, so it is a pleasant surprise to find such an affordable, friendly, clean accomodation there. Staff was very helfpul in giving advice for breakfast/dinner locations and stored my luggage for a day. Great people. They even have a deal with the posh hotel across the street so you can use the spa there for less.