Green Park
In contrast to the other Royal Parks in the area, Green Park can seem a little plain. It has no lakes or buildings and few monuments. It is still a pleasant green, lightly wooded, area in the centre of London, neighbouring two other parks and Buckingham Palace.

In contrast to the other Royal Parks in the area, Green Park can seem a little plain. It has no lakes or buildings and few monuments. It is still a pleasant green, lightly wooded, area in the centre of London, neighbouring two other parks and Buckingham Palace.
Check the official site before travelling; hours and prices can change.
More parks

Hampstead Heath
320 hectares of wild north London — swim the ponds in summer, climb Parliament Hill for the skyline.

Greenwich Park
Royal park with the Observatory at the top — best skyline view east of central.

Regent's Park
Queen Mary's rose garden in June. Primrose Hill at sunset, year-round.
Source: wikivoyage