Enjoyed staying at Katarina's apartment. Very clean, and recently refinished.
As stated in other reviews, the apartment is very close to subway station - and from there short ride downtown. There is a large supermarket near by - Edeka I think is the name - open till midnight mon-fri.
I was also able to bicycle from downtown (e.g. Brandenburg Gate) through Tiergarten Park. 26 minute ride to downtown.
I didn't explore too much around the immediate area.
I was on my own, but would be very comfortable for two. Enjoy Berlin!