On the Jesuit Meadow in Vienna's Prater, not far from the amusement park of the same name, there are two courts, which you can use free of charge and without registration. There are usually sufficient players on site, who have brought a net along and have already set it up.
It's just the same on Danube Island, where one of the oldest and most traditional beach courts in Austria is located near the Georg-Danzer-Steg (U6 Danube Bridge) - a center court with earth embankments as natural grandstands on three sides. There are two further courts without grandstands right behind. NOTE: Players must bring their own net.
Another summer oasis, the Summer Stage at the Danube Canal, has - in addition to an extremely cosy chill-out gastro area - a fenced-in beach volleyball court with floodlighting, which can be rented for a fee, if available (Reservations with Niki Kaan on tel. +43 699 1134 0171 or firstname.lastname@example.org).
The Vienna City Beach Club offers two very nice courts and an ambiance for a nice chill out, just aside the Danube river. Get a drink at the Barefoot Bar and take a seat in one of the deck chairs.
These and many more beach volleyball courts in Vienna can be found at www.beachvolleywien.at