Huoay Kwa Waterfall inmiddle of the green.
On the way to Chiang Saen one passes the village of Huay Meng, four kilometers after Chiang Khong. Leaving the mainroad by turning left before the bridge leads into the mountains. After a while the paved road becomes a dirt road. It splits up several times. Always following the main track, passing a small river (no bridge there) leads after some kilometers where one has to turn left, rolling down a short slope and stop at a riverbank in the green. There are two waterfalls. A small piece riverupwards (170 m) is Huoay Kwa waterfall with a small basin at the bottom. In rainy season it might be good for a swim.
The other one is Huoay Sai waterfall. One crosses the small river (easy jumping over the big stones) and passes through a banana field at a long slope. Later the path turns to the right. Might be the way becomes impassable there and ends up in the dense jungle if nobody cut the track free.
Another waterfall is behind a Hmong village which is 10 km away from Chiang Khong in the mountains. In Chiang Khong one has to drive the parallel road to the mainroad, turning westwards at the small petrol station. From the Hmong village it's a tough 3 hours tour following a small river upwards, first crossing rice paddies, then entering the jungle and following a jungle path for a longer time. There is a natural pool for swimming. But, that means really jungle trekking and it's good to have good shoes and a machete with you.
There are more waterfalls on the road 1155 to Thoeng via Wiang Kaen. Several signs on the mainroad point them out, but it's not easy to find them.