Spread over 1200 sq km in Champasak and Salavan Provinces, Phu Xieng Thong NPA is most accessible about 50km upriver from Pakse. The area has a sometimes other-worldly beauty, with oddly eroded outcroppings and exposed sandstone ridges, some of which contain prehistoric paintings. Most of the big wildlife has been eradicated by hunting, but there are still lots of birds, including significant concentrations of green peafowl, and a diversity of wild orchids.
The typical trip, available December to June, through Green Discovery for US$132 per person (in a group of four or more), begins in the Mekong River village of Ban Mai Singsamphan, where you will do a homestay between two days of moderately challenging trekking. A highlight is the sunset view from the top of Phu Khong (Khong Mountain). The return trip to Pakse includes a boat trip on the Mekong.
Independent travellers are pretty much out of luck since Pakse's provincial tourism office no longer arranges trips here and none of the local guides speak much English. If you get yourself to Ban Mai Singsamphan you can make it happen.