On a hot day there is nothing better to do in London then go for a swim in the Heath ponds. You get completely transported you out of the city as Hampstead feels like being in the countryside except you're in central London?!
Just down the road from where I live is this Art Deco cinema where mainly locals go. Watching films and sitting in this old theatre really adds another level to the experience.
Also the Queens back garden, these beautiful grounds are great for a Sunday stroll to see the pelicans or to feed the squirrels. Duck Island Cottage is worth a look too, a idyllic little cottage situated in the grounds which was specially built as the bird keepers home.