Meteora is the big deal stop. It is amazing to see the various monasteries high in the sky. Each one has different opening hours. It's best to book a tour as the operators will know when and where to take you. I also really enjoyed Thessaloniki, much more than anticipated. It has superb museums and a good hop on hop off bus tour. The Royal Tombs museum in Veria, an hour west of Thessaloniki, is fascinating.