Photo: Wikimedia CommonsChapultepec is the citys green lung, a huge forested park with lakes, a castle on the hill and world-class museums scattered through the trees, all a short walk from Reforma. Weekends bring families, balloon sellers and street food in equal measure. It is free to wander and easy to lose a whole day in. Rent a rowing boat on the lake, then climb to the castle for sweeping views back over the city.