Chian Saen, Golden Triagle and Mae Sae.

From Chiand Mai a bus to Chiang Rai, 2 hours + 1.5 hours to Chiang Saen, just on the border with Laos, 8 kilometers from the golden triangle where Laos, Myanmar and Thailand meet.

The place became famous cause of the production and traffic of opium, Most of the world's heroin  came from the Golden Triangle until the early 21st century. Opium museum is great, totally multimedia, just few klm from the golden triangle, but not allowed to do photos inside.