Many cruise ships stopover in Piraeus, the closest port to Athens, and cruise ship passengers can go into the city for the day on a shore excursion or independently. Most cruise ship shore excursions include a driving tour of the city, with stops at the Acropolis, Acropolis Museum, Greek Parliament, Olympic stadium, and Plaka area for shopping. A full day cruise tour of Athens can be an excellent introduction to the city for those who had not visited before. One note -- the climb to the top of the Acropolis is steep, with lots of marble steps.
Those with more time in Athens might want to do an excursion to Cape Sounion, which is on the way to the airport. The drive along the coastline is very scenic. The Temple of Poseidon, which dates back to the 5th century BC, is perched high on a hill overlooking the sea. What a great place for a temple dedicated to the god of the sea!