Meteora
~2 hr 45 min 230 km from Thessaloniki

Meteora is one of the world's most spectacular landscapes: Byzantine monasteries built atop giant, sheer rock pillars that rise as if suspended in mid-air (hence the name).
What you'll see
Of the original monasteries, six survive and function today, built between the 14th and 16th centuries. They are open to visitors, with frescoes, relics and breathtaking views. The base for a visit is Kalambaka and the picturesque village of Kastraki.
UNESCO
One of very few sites in the world inscribed by UNESCO both as a cultural and a natural monument.
Practical
It is the most distant trip on this list, about 230 km (~2 hr 45 min). It needs a full, long day (start early) or an overnight. A dress code applies at the monasteries (covered shoulders/knees; a skirt for women).