One peculiarity of the Greek plumbing system is that it can’t handle toilet paper as the pipes are too narrow and back up easily. Toilet paper, tampons etc should all be placed in the small bin provided.
Very occasionally outside the big towns you might come across squat toilets in older houses, kafeneia (coffee houses) and public toilets.
Public toilets are rare, except at airports and bus and train stations. Cafes are the best option, but you are expected to buy something for the privilege.