I'm likely headed over there in early June. I will travel around Greece some, but most of my time will spent there. My friend that lives there will be working during the day so most of my daylight hours will be spent solo. I know I'll get recommendations from her about stuff to do, but figured I'd get the non-native advice as well.
I am not sure whether I am of much help. I went there many years ago. All I can see that the city has a nice water front and one of the museum on Macedonia was interesting.
My wife and I had a 2 day stay at the Hyatt Regency Hotel in Thessaloniki. The grounds are beautifully landscaped. The terrace which overlooks the gardens, waterfalls and swimming pool is a great place to relax and enjoy a cocktail. The hotel is located outside the city close to the airport but we had no problem in commuting to the city. The staff were well trained, polite and very helpful. The Modiano Market is worth a morning visit. Check out the small restaurants, kebab shops and the Kentriki Stoa bar. The White Tower has 6 floors and a restaurant, Greek cuisine. From the balcony there is a wonderful view of the city. The Museum of Byzantine Culture is well worth a visit. The collection of Byzantine art and icons is magnificient. The Halaro Restaurant serves great food and the service is first class. We enjoyed toskana salad, roast chicken and very good wine. The Agirovoli Restaurant is a lovely seafood restaurant in the heart of the city. I had the pasta with lobster which was delicious. My wife enjoyed a shrimp and spaghetti dish. We enjoyed our 2 day stay in Thessalonki.