If you wish to go for some free tango dancing, you can do it every evening, as long as the the weather is warm and nice. It takes place in open air on Quai (or Port) Saint Bernhard, Place Tino Rossi. It has been going on for some 10 or 15 years.
The photos are far from sharp, but I hope that they show that people are really dancing.
I wish you a nice weekend!