Rooftop Pool, The Berkeley, Knightsbridge, London

In a city not known for its pools, the heated rooftop lap pool of the classy Berkeley Hotel in Knightsbridge is one of the best-kept secrets in town. Built in the 1970s and framed by handsome Roman pillars and arches, it's set under a retractable roof that opens up to the sky on warm days. You get rare views of the leafy expanse of Hyde Park to the north, and an eagle-eye vantage point of Knightsbridge and flagship stores Harvey Nichols and Harrods to the west. You can't order cocktails from the Blue Bar, but you can sip juices from the spa menu.

Tip: For US $120 (GBP 60) a day, nonresidents can get access to the pool and spa.