Classical music.
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Pałac Szustra
A neogothic palace, once a remote park residence, now nestled between residential buildings. A reconstructed "mauretanian house" signifies the entry to the vestigial park from Puławska street. The palace is home to a musical company and not open to visitors except concert nights.
The Królikarnia park is named after a pretence to being a rabbit warren for the King Augustus II in the 18th century, but in reality it served as a leisure residence and a place for meetings of the arts community. Those took place in the classicist palace, modelled after the renaissance Villa Rotonda outside Vicenza by the royal architect Domenico Merlini. The palace is situated right on the slope of the Warsaw escarpment, providing idyllic views of the greenery (and by now - low-rise residential areas) towards the Vistula river. Today, the park is open to the public and the palace hosts the sculpture department of the National Museum, dedicated to famous 20th-century sculptor Xawery Dunikowski, with several works scattered around the park. The park and palace come alive with cultural and social events in the summer months, so check the schedule in its website.
Biblioteka Narodowa
Not a major architectural draw or a typical tourist attraction, but a massive vault of knowledge and artistic work in print and other media. If this is of interest to you, you can easily schedule hours if not days for visiting the National Library.
St. Catherine's is the oldest continually existing parish within Warsaw, founded in 1238. This may be a bit misleading, as the current church building is much younger, dating in its present form from 1848, but still well-worth a visit for its elegant form, picturesque hilltop location and quite pristine condition. The churchyard contains the Monument to the Victims of Communist Terror of 1944-1956.
Pałac w Wilanowie
Wilanów Palace is tucked away in the romantic surroundings of the royal gardens. It's an ideal place to take a date or just to relax, but be forewarned that there are a few ponds and lakes along the walking paths that are infested with mosquitoes, so be wise and protect yourself or your date with bug repellent. The interior of the palace has been converted into an art museum.