Temple of Olympian Zeus and Hadrian's Arch

The largest ancient temple in the world
Word to the wise: Go to the south side of the temple from where you can see an area of current excavation not open to the public.

Hadrian's Arch stands in the middle of one of the busiest junctions in Athens. Built in 130 BC to mark the edge of the classical city, these days the rushing traffic somewhat belittles its magnitude, but nonetheless, it is worth a look on the way to the Temple of Olympian Zeus.
The temple stands mighty in a dusty clearing. Even with only 15 of the original 104 marble pillars remaining erect, it is staggering. Indeed, it was such a monumental undertaking, that it took 700-years of work before it was finally dedicated by Hadrian in 131 AD. A short walk away are the Roman Baths, which are also open to the public and on route to Syntagma Square.