Nationalpark Donau-Auen is a thin strip of natural flood plain on either side of the Danube, running from Vienna to the Slovak border. Established as a national park in 1997, it was the culmination of 13 years of protest and environmentalist action against the building of a hydroelectric power station in Hainburg an der Donau. You’ll find plentiful flora and fauna, including 700 species of fern and flowering plants, and a high density of kingfishers (feeding off the 50 species of fish).
Guided tours by foot or boat are available. From Vienna, the Nationalpark Donau-Auen is best explored either by bicycle or on one of the Nationalpark-run summer tours. Contact the park office in Vienna for more details.