The Heath is the reason London has a soul and Paris has a skyline. Five and a half kilometres up and around and back, covering the four things that matter: Parliament Hill for the view, the ponds for the sound, Kenwood for the art, and the Pergola for the oh-my-god.
Start at South End Green — coffee from Ginger & White, then straight up Parliament Hill. There's no trick to it; just walk, and London lifts itself up to meet you. From the top, drop down to the ponds and follow the chain north to the men's pond fence, which I find more atmospheric than the swimmers', though the women's is genuinely wonderful too.
Kenwood is the middle of the walk and the middle of the afternoon. You don't have to go into the house, but you should; it's free and the Rembrandt self-portrait is the quietest, best thing. Tea on the terrace after.
Then west to the Pergola, which is a secret that seems to stay secret despite appearing in every guidebook. Walk it slowly. End with the descent through the Vale of Health and the one-stop Overground back.