During my 5 months stay in Athens I could easily travel around Greece. Both with organized tour by ESN local sections and autonomously. I can say that national trasports by busses are easy and cheap for students (50% off). If you want to reach the most famous archaeological sites, such as Delphi, Olimpia and Cape Sounion you can go there with KTEL company. I also took once the train to Corinth, it is just one our trip. To reach the islands you have the Pireus port (jus 8km from the center of Athens) that takes you everywhere, and, again with 50% off if you are a student.