The Alte Donau (Old Danube) is a side arm of the Danube river. The difference to the main river is, that the water is not speeding down and you won't be dragged away, but as it is not a sea, so there is always fresh cool water. The Alte Donau is also surrounded by much green und small houses; perfectly to leave the city and its hectic aside.
If you want to enjoy a lazy sunny afternoon, this is the place to be in Vienna. Only half an hour from the city centre and easily accessible by metro, the Danube is always near to rest and chill. In the evening it is also a great place to take some photos, when the sun is setting behind the Kahlenberg (Viennas own small mountain).
It is advisable to rent a small boat and chill in the middle of the river, as the riversides can get quite crowded in summer.