The Warsaw University Library has one of the most imaginative rooftop gardens in Europe. Opened in 2002, the two-level garden planted on top of the building covers 15,000 square metres with ornamental and naturalistic plantings, including a stream and pond. From the upper terrace there are good views over the Vistula and the Poniatowski Bridge. Access is free.
It's an unexpected find — most visitors to Warsaw don't know it exists. The library building itself is notable for its facade decorated with fragments of old books, maps, and inscriptions in various scripts. The garden is particularly good in spring and early summer.