Oh wow, where should I start? If you are coming to Thessaloniki (and especially if you don't have time to do any research), try to stay at Yiannis's apartment. I have never received such a thorough guide to a location from any other host. It covered absolutely everything you'd need to know from sightseeing to restaurants to places for going out. And although my friend and I were 30 minutes late to the check-in (really sorry about that, Yiannis), he still took us through all of it.
The apartment itself is very nice, warm in the winter, and well-located. Everything we wanted to see in Thessaloniki was within walking distance. Lots of nearby restaurants as well. I hope to stay there again the next time I'm in Thessaloniki!