Not on the typical tourist path, but nowadays easily accessible via the beautiful modern subway (Evangalismos station), the National Gallery is a wonderful treat. The permanent collection is outstanding, one El Greco in particular is breathtaking. We were fortunate to see a Picasso traveling exhibit while we were there. The gallery is close to the Cycladic museum, full of magnificent finds, and also has traveling exhibits (we saw a wonderful Dali show). In addition, the War museum and the Byzantine/Christian museum are just down the street. All are great more "modern" diversion if you have time... after seeing the ancient ruins. The National Gallery is well worth a visit for art lovers -- a great site not as well known, but just dwarfed by the Acropolis.
If you own or manage National Gallery - Museum Alexandros Soutzos, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.