Above: The Acropolis stop could be a bit closer to its namesake and its quicker and more fun to walk there from Plaka or Syntagma. Graphically It appears closer than it actually is. So unless your hotel is over there its more fun to walk from Exarhia, Syntagma or The Plaka to the Acropolis.
Right The Greek Agora and Stoa of Agrippa Metro route between Monastiraki and Thession Stations along Hadrian St. Extension.
Hadrian St. Extension is a nice walk you should take to see the Pnyx and stroll the Thession areas back streets with restaurants, cafes, and night action.
Many singles strut their stuff here to while away the hours as couples and families take the air in the evenings cool period around 10:30 PM. A good promenade space.
Additionally dense crowds and perhaps a pickpocket or two means its good to have your wits about you in Omonia Sq especially at night. I cant think of a busier place in Greece really. This is Athens equivalent of Times Square in NYC perhaps. While on the subject: pickpockets are not unkown on the trolley and in dense crowds. The photo above right show the left luggage lockers in Omonia Sq Station with a 15 day max.