In Athens, see the Parthenon atop the Acropolis, Temple of Zeus, tour the Agora, shop in the Plaka, see the changing of the guard at Syntagma Square. I would also see the old Olympic Stadum. Drink Retsina and Ouzo. Eat some of the best food on the planet.
Athens is not a city to be missed. If you have only a day, hire a good guide so you can pack in as much as you can. Be prepared to walk. Be prepared to sweat. Be prepared for one of the best days of your cruise.
There is nothing to see in Pireaus. It is a port with lots of ships. Nothing there. You need to go to Athens and spend the day there. I understand that there is a subway from Pireaus into Athens and it takes about 35 minutes. There are also taxis at the pier that you can take into the city.