This 2,5-kilometer-long island paradise lies hidden away in the middle of the Budapest cityscape, surrounded by river Danube. It is one of the city's most beautiful parks.
The vast green area hides a medieval ruin, a rose garden, a swimming pool, and even an open-air theatre. It is a popular destination both for recreation and sports.
Wikipedia https://en.wikipedia.org/wiki/Margaret_Island
More information http://www.aviewoncities.com/budapest/margaretisland.htm
Coordinates 47°31'35.192" N 19°2'52.154" E