Arbutus or strawberry tree. In Palaios Panteleimon, on the slopes of Mount Olympus, they make the fruit into tsipouro; you may also see arbutus honey, μέλι από κούμαρα (meli apo koumara).
On Kythera, they make a bite sized sweet from ther fruit called koumara κούμαρα
Image by Dana Facaros