More a glorified set of rapids than a waterfall, but oh, how glorious it is. The largest and by far the most awesome waterfall anywhere along the Mekong, Khon Phapheng is pure, unrestrained aggression, as millions of litres of water crash over the rocks. While pricier than the similar Tat Somphamit, this place, with its gardens and walking paths, is more attractive. You can also get down closer to the rapids.
There are several viewpoints in resort-like grounds, plus many restaurants and snack shops. With luck you can catch some rainbows in the early-morning mist. And like all the waterfalls in this area, there's a shaky network of bamboo scaffolds on the rocks next to the falls used by daring fishers. A free shuttle runs continuously between both ends of the park – a 500m trip.
Be sure to check out the pavilion for the legendary Manikhote tree, which is 150m or so in from the entrance. You can't miss it, actually.
Khon Phapheng is on the eastern shore of the Mekong near Ban Thakho. From Ban Nakasang it's 3km out to Rte 13, then 8.5km southeast to the turn-off and another 1.5km to the falls. A tuk-tuk from Nakasang costs about 50,000K return with an hour's wait time. You can also easily motorcycle down from Don Khong. The falls are included in kayak tours out of Don Det and Don Khon. Since amateurs can't kayak anywhere near these falls, you'll be taken there by vehicle as your kayaks are driven up to Ban Nakasang.