One of the highest point in London, Hampstead Heath is a massive London park, covering 720 acres in total, and one of Londoners favourite place to relax especially during Summer (other than that it's pretty windy and cold, at least for me). The coolest thing for me about the Heath is that it has not one, or two, but three(!) 'open-air' public swimming pools. Although I wouldn't call it 'pools' cause they are really are ponds, used to be reservoir for drinking water. Definitely not for the squemishes, as you will swim along with the fishes, the ducks and the swans ... or what ever lives in there :) Apart from that, the vast green area is just breathtaking.