Yoga, Tai Chi, watching TV reading a book.
I go to the sauna. I am sorry to tell you though your saunas here are not up to the standard they are in Germany.
There are lots of Sauna's in Berlin. I used to go to a family one outside of Berlin in Ludwigsfelde.
Check it out...Kristall Saunatherme Ludwigsfelde (Germany): Top Tips Before You ...
Word of warning you have to be naked for the sauna although not for the other activities.
I have to wear a swimming costume in the saunas in the UK....I mean come on...why on earth would one want to sweat in a swimsuit.?