Pingyuan C.
Google
The birthplace of Fryderyk Chopin. Summer is recommended, which is also the concert season, visitors get to listen to free outdoor concert. Very beautiful place, and very tranquil compared to any other parks in Warsaw.
The place is tricky to access by bus, please refer to the official website (not 100% accurate though) instead of listening to Google, which might get you stranded at the train station.