A visit to Athens is not complete without seeing the Acropolis. During spring and summer months it is always hot and sunny so make sure you have plenty of water and sun screen so you don't collapse from a heat stroke during your climb up the hill to see the archaeological remains. But the views of Athens when you make it to the top, and the impressive structure of the Parthenon are all worth it.
Another museum I personally enjoyed is the Benaki Museum which is a short walk from Syntagma Square (http://www.benaki.gr/index.asp?lang=en). It houses a privately owned collection of Greek works of art from the antiquity to the modern times while at the same time illustrating Greece's history.
Other than that enjoy the many cafes and restaurants (souvlaki is a must!), go shopping in Monastiraki and just enjoy the atmosphere.
|Ancient ruins underneath the Acropolis museum|
|Theatre of Dionysus|
|The Propylaea, entrance to the Acropolis|
|The Erechtheion, temple dedicated to Athena and Poseidon|
|Views of Athens from the Acropolis|
|Syntagma Square and the Greek Parliament|
|Changing of the Guard|
|Tomb of the Unknown Soldier|