I fully recommend "The λamda Project". I am Greek myself and my Japanese husband studied Greek here. We were both very satisfied with the fast progress of his language skills.
The teachers are all highly educated, always encouraging and helpful. The textbooks are modern and easy to follow. The school is located conveniently in the centre of Athens, in a beautiful, newly renovated building, with spacious classrooms and an extensive library.
The intensive courses are really suitable for those who visit Athens for a short period of time and want to combine tourism with education. Students can additionally follow classes on Greek history, mythology and philosophy, either in English or Greek. There are also Ancient Greek lessons, cultural events, workshops and seminars on various topics, such as theatre and arts, psychology, sociology and education. 10/10, highly recommending!