My girlfriend and I had a wonderful stay at Reuben and Jekatarina's place, where we stayed for a whole week. The apartment is the perfect size for a couple and is very cosy. Reuben and Jekatarina's were really helpful, and made sure that we weren't missing anything.
The apartment is very central and shopping, bars and U-bahn are all within walking distance. The nearby Tempelhof airfield is a must-see.