For generations, Londoners have headed to Hampstead Heath to escape the dirt and noise of the city, and this unique 791-acre expanse of rus in urbe ("country in the city") is home to a variety of wildlife and habitat: grassy meadows, woodland, scrub, wetlands, and some of Europe's most venerable oaks. Be aware that, aside from the southern slope of Parliament Hill and Golders Hill Park, it is more like countryside than a park, with signs and amenities in short supply. Pick up a map at Kenwood House, or the Education Centre near the Lido off Gordon House Road, where you can also get details about the history of the Heath and its flora and fauna. An excellent alfresco café near the Athletics Field serves Italian food.
Coming onto the Heath from the South End Green entrance, walk east past a well-equipped children's adventure Playground and Paddling Pool, turn left, and head to the top of Parliament Hill. At 321 feet above sea level, it's one of the highest points in London.
You'll find a stunning panorama over the city. On clear days you can see all the way to the South Downs, the hills beyond southern London.
If you keep heading east from the playground instead, you'll come to the Lido, an Olympic-size outdoor unheated swimming pool that gets packed on all-too-rare hot summer days. More swimming options are available at the Hampstead ponds, which have been refreshing Londoners for generations. You'll find the "Mens" and "Ladies" ponds to the northeast of Parliament Hill, with a “Mixed” pond closer to South End Green. A £2 donation is requested.
Golders Hill Park offers a good café, tennis courts, a duck pond, a croquet lawn, and a walled flower garden, plus a Butterfly House (May–September) and a small zoo with inhabitants including lemurs, coatis, and rare birds.