Hi there,
I recently moved to Athens, but had previously wanted to learn as much Greek as possible in the shortest space of time. Hence, the best course I could find was this one....
http://www.dikemes.gr/The nice thing (apart from great teachers and material) is that it is based on the island of Paros in June each year. The downside is that it is quite expensive (~1700EURO). However, for three weeks (inclusive of accomodation, lessons, trips etc) it was great value I think....
+++ There is also an option for a course in Athens.
The teachers include the authors of a common fav. Greek Language Book "Ellinika Tora" (Greek Now).
Hope that helps.
H.