Leafy, partly residential Żoliborz is known for 19th-century Warsaw Citadel, formerly a sprawling Russian barracks. Today it’s the site of several museums, including Muzeum Katyńskie, which pays tribute to Polish soldiers executed in 1940. A lake lined with willow trees sits in charming Park Kępa Potocka, and cycling and jogging tracks line the Vistula River, where urban beaches and easygoing bars open in summer.