Check out Golden Triangle where Thailand

The
Golden Triangle is a convergence point of the Thai, Burmese and Laos borders and one of Asia's two major opium-producing areas. Located at the junction of the Mekong and Ruak rivers some 18 miles north of Chiang Rai, The Golden Triangle is a popular tourist attraction often visited on day trips from Chiang Mai or Chiang Rai.
Along the river feature remains of numerous ancient sites and structures indicating the glorious past settlements. This is also the area where various legends concerning the Lanna ancestors originate. The opium museum is another well-known site housing some interesting displays depicting opium dens. Arrays of shops are apparent along the sloping main road - mostly catering to tourist buses which call in all day long.
It is worth mentioning that the views across the river to Laos and Burma are spectacular with a number of nice open-air restaurants boasting a panoramic vista. Border crossing is not possible at this point and there are very few structures on the Laos side.
Getting There:
Travel approximately 9 kilometers north of old town Chiang Saen, along the road parallel to the Mekong River to the area where the borders of Thailand, Myammar and Laos converge. Boats can be hired to view the upstream scenery of the Golden Triangle and to travel downstream to Chiang Khong. The trip to the Golden Triangle and Chiang Khong takes approximately 40 minutes and 1 hour respectively, depending on river currents and water levels.