A brilliant idea!
If you have time, before you reach Penteli mountain a Sunday morning (from 10-18), you could visit the Emfietzoglou Gallery.
It is located at Anavryta area, near Maroussi. The works are representative of a wide range of artistic expression, from painting, sculpture and printmaking as construction, photography, video and installations. An overview of Greek art from the 19th century until today.
(From Kifissias avenue, you turn right to Pentelis str. After 5 streets turn left to Efkalypton str. In the end of Foinikon str. you 'll find the Gallery).