Swinging from the rafters
On the river there are lots of little bars, at some you can slide along ropes or even jump from trapezes into the water, just watch your entry and don't slam into someone swimming below, often people are pretty legless there so stay safe.
Kayaking with TCK advanture tours
TCK seems to be the cheapest agency to offer trips on the Nam Sang. For a full day trip, starting with the elephant cave, tubing into a cave nearby, lunch and kayaking I paid 90000 kip. Compared to other agencies that is quite a bargain, as they offer exactly the same package for about 30 US$! Don't pay more just because it says "eco" or "green". There is no difference between the tours, they are all the same.
They provided a free drybag and lifejackets, and i could not see any difference concerning security measurements.