Athens is smaller and more manageable than most European capitals, with the major sights concentrated in a walkable central area. The Acropolis is non-negotiable — arrive at opening time (8am) to see the Parthenon in golden morning light before the tour groups arrive. The Acropolis Museum at the base houses the original sculptures in extraordinary context.
The Ancient Agora (marketplace where Socrates taught), Temple of Hephaestus (best-preserved ancient Greek temple), and Hadrian's Library are all within walking distance. Plaka's winding streets, Monastiraki's flea market and street food, and the hip Psiri neighborhood for evening life complete the picture. Day trips to Cape Sounion, Delphi, or Corinth are popular.