Even if Athens is full of history, the main tourist attractions can be seen in a day. This is because they are not that many and are conveniently located in the same place.
The Acropolis is the main tourist landmark in Athens. This was the old city center. It is placed on a hill, so be prepared to climb it. Take enough water with you during hot weather. The Acropolis is full of ruins and history, so you might want to spend some time here. It also offers a great view of the city. Once you get to the top, you will be impressed by the temples: the Parthenon, the Erechteion and the Temple of Goddess Athens.
The Temple of Dionysus is well preserved and is also located close to the Acropolis.
The Parliament is located in Syntagma Square. It is a nice building, but the most exiting part is the changing of the guards. Their traditional uniform, especially their shoes, will definitely make you smile.