Introducing Temburong District
This odd little exclave (that means a part of a country physically separated from the rest of the nation; feel free to take that to the next pub quiz night) is barely larger than Penang, but happens to contain one of the best preserved tracts of primary rainforest in all of Borneo. The main draw is the brilliant Ulu Temburong National Park, accessible only by longboat.
The speedboat ride from BSB out to Bangar, the district capital, is the most fun you can possibly have for B$6. You roar down Sungai Brunei, slap through the nipah-lined waterways and then tilt and weave through mangroves into the mouth of Sungai Temburong.
Want to get depressed? Go to Google Earth and look at the outline of Temburong District. It's easy to spot: at the Brunei frontier, Malaysia's logging roads – irregular gashes of eroded earth – and trashed hillsides give way to a smooth carpet of trackless, uninhabited virgin rainforest. Until not long ago, almost all of Borneo looked like this.