It would be possible to spend all day meandering the paths that link the Royal Parks around Buckingham Palace. St. James’s Park has grassy knolls, hidden ponds and the Horse Guards Parade, plus a good café at Inn the Park, and opens out onto the forecourt of Buckingham Palace. (You know the Queen is at home when the flag is flying.) Green Park has more open areas, deck chairs and picnic spaces. The two are separated by the Mall.