The roof gardens of the University of Warsaw Library are certainly a worth place to visit. The entrance is completely separated from the main library entrance, and is free. Just additional points :). And a lot of stuff to admire. There is something for plant lovers and for architecture lovers as well. I will probably post a few more images from that place, but my favorite is one of the reflections images. There were quite a few of them, as much of the construction is made of glass. But this one is by far most stunning.