Danube Island near Porto Pollo Harbor

Definitely recommended (even on rainy days).

Vienna Danube Island - insights and perspectives
Approximately 21 km long, located between the Danube and the new Danube. This is where the Viennese go on weekends. They grill, chill, fly kites, sunbathe naked, go for walks, rollerblade and cycling and so on and so on.
We were also thrilled. It's just lovely to cycle along the many paths. Grab a coffee here and there and then you're on your way. By the way, the Viennese are very sociable and nice. Helpful, of course. If you stop to look at the map, they'll ask if they can help. We've only been here for 2 days and have had these questions at least 6 times. I like these people. They're really relaxed. And you'll be on this island too.

And I find the eastern half even more beautiful and naturally more versatile. I especially enjoyed this route