You'll find several Thessaloniki travel guides in bookstores (or can be ordered online);
You can also visit the city's official website:
https://www.thessalonikitourism.gr/index.php/en/
We spent four days in Thessaloniki last September. It's a very different city from Athens, with a predominant Ottoman and Byzantine influence. Culturally, the people have a different mentality than in Athens.
Since we tend to wander around the city, sometimes to places off the beaten track, we could have easily stayed two more days.
The "modern" part by the sea is nice, with its famous White Tower and the large Aristotelous Square (designed by a French architect), but we preferred the old town (Ano Poli) in the hills. Public transportation is efficient, and there's now a metro line that opened this year.
I can't recommend a hotel as we're staying in an apartment.
Some pictures:
https://photos.app.goo.gl/qhhJT51QRDaLK41N7