The trip to Corinthos, Mycenae and Epidayros is very nice idea, if you are interested in archaeological places. The theatre of Epidayros is really perfect and it has the better acoustic from all the theatres in the world. In Mycenae you are going to visit the ruins of the palace of Agamemnon which is very impressive. And the ancient Corinthos is very nice.

On the other hand, Acropolis and its Museum and the area around (ancient Agora and it's museum- Stoa Attalou, Thission etc) and Plaka will occupy a whole day. You need more than 5 hours to see in a hurry the National Archaeological Museum and there are other 3 important museums you have not to miss: Museum Benaki http://www.benaki.gr/ , Museum of Cycladic Art http://www.cycladic-m.gr/ and Byzantine Museum http://www.nyloo.com/html/ent/661/ent.16661.1.asp .

A web page with all Athenian museums is the following http://www.athens-today.gr/Museum/museum.html

A general site on what you can see while in Athens is the official site of Greek Ministry of culture: www.culture.gr

In you need more specific infos, please don't hesitate to ask me.

Best Regards Joanna in Athens