This simple souvlaki restaurant has been running since 1964. It is in the pedestrianised Monastirakis Square in central Athens, yards from a large market, along with a host of other restaurants and cafes. In Ottoman times the square was a central grain market, with the grain sheds built over the ruins of the Library, with a Byzantine church at the heart of the square. These days it is all cafes and shoppers going to the nearby flea market.
O Thanasis has a dining room but the main action is outdoors, with tables just outside the restaurant and also opposite, across the street. The speciality is souvlaki, but you can also eat a kebab or a gyro. A Greek salad was generous in scale and pleasant enough, as well as being literally a third the price of one of a similar standard that we tried the day before at a well known Athens hotel (11/20). My chicken souvlaki was just a touch dry, but had decent flavour, served on a flatbread with a simple salad and chips (11/20). Service was functional but cheery enough, and the bill just for one course and a drink came to €11 apiece. The toilets are downstairs, and were not an advertisement for the restaurant on the day of my visit.
Of course this is very simple food, so hardly somewhere to which you would make a special excursion, but if you are in the Monastiraki area of Athens and fancy a bite to eat in the open air then you could do a lot worse. The place has been running for over fifty years and counting, so clearly has hit on a successful formula.