Such a cool thing to do, just wandering around the ancient grounds of former marketplace is unbelievable. lots of different ruins to see and different levels of preservation, highlight is Temple of Hephaestus and the museum area.
Best to take your time and slowly walk through ancient ruins as so much to see, some areas that you can sit and relax, this is part 2 of the acropolis complex and its certainly worth a few hours to fully take in all ruins and artifacts.
I would put this in top 5 things to do in Athens.