In the corner of the cafe. The G 1.5 octaves lower than middle C is mute, and the C 1 octave lower than middle C always resonates. A functional and usually available piano but not a great one. People here will recommend you try to get a time to use the piano far downstairs, which music students reserve and use (and which is sometimes is used for concerts)
It is good to ask crew first if you can play.
not a grand piano
Przy wejściu, przed bramkami jeszcze obok kawiarenki
Sometimes it's key-locked and to have it unlocked just ask in nerby coffeeshop