PULAU UBIN, Singapore — It takes no more than 15 minutes to make the eastward crossing on a juddery old bumboat from Changi Jetty on Singapore’s main island to Pulau Ubin, where gray-barked pulai trees stretch skyward, their pillar-straight trunks evoking the slate and glossy office towers that crowd the Singapore skyline. The 1,020-hectare boomerang-shaped Pulau Ubin is “the last kampong,” or village, and “a living showcase of what Singapore was like in the 1960s,” according to Visit Singapore, part of the country’s official tourist board. Not surprisingly for a place advertised as such, the short boat trip across the narrow strait aims to take visitors back a half century. Pulau Ubin, or Granite Island, is a preservationist’s pearl — a verdant throwback to the pre-industrial, pre-urban way of life still to be found here and there in rural Malaysia and Indonesia. Those old ways are otherwise history in Singapore, where 5.6 million people are jammed onto a mere 720 sq. km. of land area.