Farther south from Xishuangbanna, you can take a cruise boat and visit three of China's neighboring countries. A speedboat will take you down Lancang River, known as Mekong outside China.
You don't need to bring your passport, unless you must have special clearance for international travel. On your right side is Myanmar and on your left, Laos. Short getaway trips can also be arranged.
But the real beauty is when you arrive at Thailand's Chiang Saen, near the center of the Golden Triangle and where the opium trade used to flourish. From there you can hop on a bus and it's just a few hours to Chiang Rai and Chiang Mai in Thailand, or Luang Prabang in Laos.
You can also take the cruise back to China. But to see more of Southeast Asia, you can opt instead to fly from one of the nearby cities or ride the train or bus.
It's better to make your trip coincide with the rainy season - boats can get stranded when the water level is too low.