Well, it's an agora. There was all the equipment that an old Greek city would need: a library and temples for various gods of which 'Temple of Olympian Zeus' is the most attractive today. There is a distance between 'Roman' and 'Greek' agora, I am sure that there was an ancient street once upon a time between them.
At the Roman Agora, Tower of Winds should not be missed. This cute building was built by a Syrian scientist to make some measurements about air, temperatures, water, etc. There were, of course, water sources here once upon a time. One of the presents of the ancient gods to Athens was water by Poseidon and olive trees by Athene; the second one won. In the Greek agora, an old church from 11th century and the headless statue of Hadrian were favourites.