Return to Athens and spend the day exploring the city's many cultural attractions, including the Acropolis, the Parthenon, and the Agora. End your trip with a farewell dinner at one of the city's many excellent restaurants.
This is just one possible itinerary, and there are many other destinations in the Saronic Gulf and around Athens that you could visit. You can customize your trip to suit your interests and preferences, and add or remove destinations as you see fit.