Athens is an amazing city where you need at least two full days to explore, three if you like ancient artifacts and buildings. You will need a minimum of two days there and since you arrive in the morning, that counts as a day as long as you don’t check into your hotel and go to sleep. Do not miss the Acropolis or the Acropolis Museum. And buy your tickets in advance. I like https://www.getyourguide.com/ or you can buy directly for 10 Euros through https://hhticket.gr, keep in mind that they are now enforcing time slots.