In summer in Vienna there is a place in Stadtpark called the Kursalon. Every evening there are open air concerts by a waltz orchestra, and there are dancers there dressed in waltz costumes to show you how the correct Viennese waltz is danced. Then they open up the dance floor for everyone to give it a try. It's romantic waltzing in Vienna under the stars. I'm not sure how far into September this goes. Perhaps the concierge at your hotel could help.