Carry your passport with you at all times in case you’re stopped by the police and questioned. Greek citizens are presumed to have identification on them and the police expect much the same from foreign visitors.
Greek drug laws are the strictest in Europe. Greek courts make no distinction between possession and pushing. Possession of even a small amount of marijuana is likely to land you in jail.
If you’re arrested insist on an interpreter (διερμηνέας; the-lo dhi-ermi-nea) and/or a lawyer (δικηγόρος; the-lo dhi-ki-go-ro).