Athens is an interesting city and much less dirty, crowded, and noisy than most people seem to believe, but its attractions are few and far between (great archaeological museum, though). The one exception, though, is of course the Acropolis. It can be seen from almost everywhere in the city and is probably the greatest surviving ancient monument anywhere. Its WH listing says that it symbolizes the idea of world heritage, and that's certainly true. Not only its architectural, cultural, and historical significance is enormous, but it can also be considered to be the birthplace of democracy. So walk up the hill (not very strenuous), enjoy the view, see the temples and the museum, and be aware that everything that makes Western civilization unique started from here. An unmissable and unforgettable experience.