Municipal Gallery of Athens Gazi, Keramikos & Metaxourgio

This city-run gallery has temporary exhibits, sometimes featuring Greek artists but not always. It's definitely worth a peek as it's set in a grand mansion that's had many incarnations, including as the silk factory for which the neighbourhood of Metaxourgio is named. Danish architect Christian Hansen (brother of Theophil, responsible for the landmark Athens Academy building on Panepistimiou) designed it in 1833.