The most interesting and popular thing to see around Si Phan Don is the heavily endangered Irrawaddy dolphins. Often seen by local people as humans in another incarnation, they have a mystical significance. The south side of Don Khon is where you will find them. April to May when the weather is dry and the river is low is the best time of year to see these elegant creatures breaking the surface of the water often in pairs. Boats can be chartered for about $10 an hour in the village of Ban Hang Khon on the island's southern tip.
The Khon Phapaheng Waterfalls (admission $1) is the stretch of water that confounded French trade ambitions on the Mekong and when you see them you understand why. A huge volume of water surges powerfully downstream to Cambodia. When the rains are at their height and the river is high, it is a very impressive site. The Khon Phapheng falls are of local religious significance, being seen by Buddhists as a place that spirits become trapped. The same is true of the Somphamit Falls on the western side of Si Phan Don. Here you can also see local fishermen trapping fish from precarious wooden causeways using huge bamboo traps. The water is forced powerfully through the narrow walls of the rocky gorge through which it is flows.
The French may have had their trade ambitions stolen from them by the force of nature but in small way they tried to ameliorate the mighty force of the Mekong around Si Phan Don with a small railway across the islands designed to haul a modest amount of goods from Cambodia across the islands from pier to pier. There is still the remains of a rusting locomotive. You can follow the trail of the old track across the islands.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.