One of London's 10 royal parks (Primrose Hill, Regent's Park, Kensington Gardens, Hyde Park, St. James's Park, Brompton Cemetery, Bushy Park, Richmond Park and Greenwich Park being the others that make up "the lungs of london", 5000 acres managed by the Royal Parks Agency). Less well known than Hyde and St. James's parks, which it links to form a continuous open space in the heart of the city, it lies just to the east of Buckingham Palace. It is often a more peaceful environment than those larger parks, an effect enhanced by its tree-lined avenues and the fact that it eschews flower beds and shrubberies for grass and mature trees (Beech, Lime and Plane) making it the largest green space in London (despite being, at 60 acres, the smallest of the royal parks).
If St. James's Park is Buckingham Palace's front garden, then Green Park was its duelling grounds- a Welcome to London article notes that "many a gentleman, defending his or somebody else’s honour, died on the turf where office workers now enjoy their lunchtime breaks". These days a picnic is more likely than a duel, and as a result ice cream stands and deckchairs are usually available in the summer. Also of note is the Diana, Princess of Wales memorial walk, the genuine gas lamps (with clockwork timers) and Constitution Hill (so named for Charles II's love of taking walks or "constitutionals" there). A 41 gun salute to mark the Queen's official birthday is held in the park each year on June 16th, by the King's Troop Royal Horse Artillery.
My own experience of Green Park was thanks to the questionably acronymed JuMPinG (Junior Mensa Picnic in Green Park), which grew out of Mensa's picnic in the park after various JMs failed to find that event and so just held their own. The fact that they came back the next year goes to show what an ideal place Green Park is for meeting up and just relaxing- the vast green space allows everyone to spread out, unwind and forget they're in the city- unless the weather lets you down, in which case all the delights of central london are at hand (which may explain why we ended up playing twister outside the royal society of chemistry).
Green Park is served by, strangely enough, Green Park underground station which, if you get it right, exits straight into the park (of course, I got it wrong and had to walk around to one of the gates, but nevermind...). This zone 1 station is the starting point for the new section of the Jubilee line and the Piccadilly and Victoria lines also run through it. Park entry is free and there is often live music from the bandstand.