The Roman Agora in Athens is much smaller than the nearby Ancient Agora, but it is still an important archaeological site to visit. It was built between 19 and 11 B.C. and was a commercial center in ancient Athens. The Tower of the Winds is located here and is arguably the primary attraction.
I couldn’t take photos inside, but the National Library of Greece is gorgeous. A couple were posing for wedding photos on the front stairs while I was visiting.