Navarino Square, Thessaloniki
It's a nice cafe in Navarino square which has the best fruit juices and fruit based cakes and salads in Thessaloniki.
Best time to go there is during the afternoon, but not long after 2 o'clock cause it would be difficult to find any table available.
By going to Froutotypo in the summer, you can also get some hints about whether to stay at Thessaloniki or not. For example: in case you go there at 3:30pm lets say and the cafe is empty, then you are at the wrong place at the worst possible time. Meaning that you should better go to Halkidiki and by the beach.