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 every second. That said, you'll have to decide whether or not it's worth the jacked-up price since you can see the smaller but similar Tat Somphamit for a lower price and without the tour-bus crowds.
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 fishermen. A free shuttle runs continuously between both ends of the park – a 500m trip.
Khon Phapheng is at 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 săhm-lór from Nakasang costs about 50,000K return with an hour 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.