National Garden is a marvelous park, open to the public from sunrise to sunset, located in the heart of Athens, just behind the Greek Parliament. What were once, the Old Palace and the Royal Garden, the getaway of Queen Amalia and King Otto, is now a delightful, shady refuge for Athenians and visitors during the hot summer months.
http://www.thisisathens.org/explore/venues-attractions/12-attractions/national-garden-zappeion/