With its scenic central pond, tended flowerbeds, and picnic-friendly lawns, it's difficult to believe that this Royal Park was once a swamp near a leper colony. Today it's as elegant a green space as London can muster, and one of the best places in the center of town to watch wildfowl. Its pond is home to more than 20 species, including ducks, geese, and even pelicans -- the descendants of a pair presented to Charles II by a Russian ambassador in 1662 -- which are all fed daily at 2:30pm.
Great as the park is for just lazing about, if you want to be a bit more active, the park office offers guided walks throughout the year -- including to Duck Island in the center of the pond, on the hunt for the park's bats, and following the course of the River Tyburn, which flows beneath the park. See the website for times and frequencies.