While in Greece (specifically in Athens), try to make it to Meteora which is known for it's monasteries that are literally built on top of rock formations. Meteora itself is actually a UNESCO World Heritage site.
There are currently 6 monasteries remaining of the original 24 that are built on natural rock pillars that you will stare at and wonder how it was made.
We visited this on a 2 day tour and it was a good enough time spent being there in my mind. On the 2 day tour you stay at Kalampaka where you will actually be able to see the monasteries if you have a good enough eye.
The tour itself consisted of us visiting the Great Meteoron, the largest of the monasteries and St. Stephen which is a small monastery, but is actually only for females. Both places are very interesting to see, especially when you look and wonder how it was built and when you are inside, it is as if you were just in a regular monastery.
Well worth the visit in my mind, although I would guess that you will need 2 days to visit it since it is a 4 hour drive from Athens.
Come back next time for another Past Point of Interest!