In 2012, I took two trips to Greece, first to Kavala in August, then to Kavala, Meteora and Thessaloniki in October.
Kavala is a picturesque port in Northern Greece, close enough to Istanbul to be a long weekend trip by car. It is strung along the coast with hills behind it. The old part around and above the harbor is filled with restaurants, which serve excellent, if a bit touristy seafood. I made the mistake of driving up there with my car, and almost ended up being permanently stuck in a really, really narrow street.
Meteora is kinda in the middle of Greece and it is justly famous for its rock formations and monasteries that are perched on them. There are six monasteries, and you can visit all of them. (I think. I visited three of them, and that was enough. Once you have seen three beautiful monasteries with drop-dead-gorgeous views, you have seen them all.)
Thessaloniki is another beautiful port city in Northern Greece, and it has special meaning for me as the birthplace of Ataturk. (I didn't visit the house he was born, however. Next time.) It is exactly the right size for a city, neither too large nor too small and reminded me of Izmir. There are an unreasonable number of street cafes and restaurants, and the entire population of the city seem to be eating or drinking at every hour of the day. My kind of city.
It has a good Archeological Museum, and the Museum of Byzantine Culture is also very impressive. The Citadel has a good view and several nice restaurants on the way up with great views.