Meteora, in central Greece, is a place of natural beauty and man-made wonder. The huge natural rock towers are home to cliff-top monasteries built by Eastern Orthodox hermit monks in the 14th century. The monks settled in the area from the 9th century and began by living in the caves and fissures of the rocks. They built the inaccessible monasteries - 6 of 20 survive - to fend off Turkish invaders. UNESCO has identified the area as of world significance.
You'll need a bit of time to explore Meteora and you can stay overnight at Kalambaka, a modern town since the old one was burned in World War II, or smaller Kastraki closer to the rock. There are guided tours from Athens or buses and trains run to Kalambaka. From Kalambaka there are buses to Meteora.
Be warned, there can be many steps to climb and there is a dress code appropriate to religious buildings.