I am a yoga-lover and this must be one of the most beautiful spots to practice: the roof terrace of the Volkshotel (only when the weather allows it though, otherwise classes are inside). Classes take place at Monday or Wednesday evening (7pm) and Sunday morning (10am). All you have to do is send them an e-mail to sign up and pay 10 euros to the teacher. Namaste.
Wibautstraat 150, 1091 GR Amsterdam, Netherlands