Thessaloniki

Rick probably has nothing for you, but the city itself can help you!

https://thessaloniki.gr/visitor/?lang=en

And if you don't have much time, I suggest the Cultural Route bus which leaves from the White Tower

http://www.oasth.gr/wp-content/uploads/2023/04/BROCHURE_EG_03_10_22.pdf

FYI - if you are a enjoyer of Gyros, it is common for gyro pita's in Thessaloniki to be dressed with ketchup and mustard. If you would rather have tzatziki instead you can ask for it.

I haven't been to Thessaloniki in a decade+ but I would visit the Rotunda, the Arch of Galerius, the Roman Forum, and of course the White Tower. All the old sites can be visited in a couple of hours. The best thing to do is just stroll around and take in the city imo. And get some bougatsa

There are also a couple of good museums: I enjoyed the Archaeological Museum of Thessaloniki and loved the Rotunda.

Depending on how long you have I'd probably try to get a local guided walking tour to see the most in the time you have. Or you could do some research into the sights and decide what you want to see and design your own tour. I enjoyed the White Tower, the bits of old city walls, the Greek Orthydox Cathedral, and just wandering the streets of old town.

I second the idea of just doing a walking tour when you are somewhere for a day. We had lots of little stops on our last trip to the UK and it helped immensely. We're coming to Greece and the fall and will do the same thing.