Mentioned by Lonely Planet
20 best free things to do in London
"This enormous, ancient parkland is one of the best places to escape the city while at the same time catching an amazing view of it: the vista from Parliament Hill, which forms the southeast part of Hampstead Heath, is so impressive it’s actually protected by law. Elsewhere in the park you’ll find a zoo, three swimming ponds (nominal charge, which is often ignored) and plenty of quiet spots for a back-to-nature-in-the-heart-of-London picnic."