ShotHotspot has mapped 20 abandoned places in Nomarchía Athínas, Attica, ranked by how often they're photographed, viewed and voted for. The most popular are Stoa of Attalos, Abandoned Athens Ex!t Primitive and σταθμός λαρίσης アテネ εξηκίου. Some are private property or unsafe, so get permission before you visit.
We don't know whether you're allowed to visit any of these places. Many are private property, closed to the public or unsafe: get permission from the owner or the relevant authority, and take safety precautions.