Very good flat. Central, close to nightlife but in a quiet street, clean, spacious, sunny and very airy (has 2 balconies). Nice view from the front balcony, although it's a little small, even for just 2 people. Good shops around for getting some breakfast. Mr. Athanasios is very welcoming and will try to make your stay very easy with tips about the city. A taxi stop is also very close.
The only disadvantage is the small elevator (if you have a lot of luggage), but this is very common in old buildings in the center of Thessaloniki.
Recommended!