There are a few buses and minivans from Pakse that include a boat ride to Si Phan Don. If you are travelling from Cambodia, you can take a van from Kratie, Stung Treng or Banlung for about 15$, which also include a boat ride to Don Det or Don Khong.
Kuang Si Waterfalls
There are many options to get to the waterfalls: by tuk tuk, motorbike, car, bicycle or by boat down the