Visit the White Tower
A good idea is to visit the White tower. It is a guard tower made by the Othomans in the end of the 15th century. Now it is a small museum with historical and archaeological interest.
This tower has been used from Ottomans as a prison till the end of 19th century.It was called "Tower of blood".Now it works as a museum
Inside the White Tower a small museum hosts some outstanding valuable articles. Here is a couple of bracelets of gold and enamel, dated on 10th century AD.
It is probably the most famous building in central Thessaloniki. The White Tower used to be a prison (and it was called the Bloody Tower then). At present it hosts a small museum of byzantine art.
Thessaloniki's most prominent landmark is not white anymore but houses an interesting Byzantine museum. It's open Tuesday to Sunday and free of charge.