Gunnar Caya
Google
These might not be the largest or the most powerful waterfalls in Si Phan Don, but in my opinion they were the most beautiful. So many different viewpoints and sections of the fall to see. And getting to these areas was easy, just a relatively short walk along well maintained paths.
If you keep going, you eventually get to a really neat beach. You aren't permitted to swim there, but it is a super relaxing and beautiful place to unwind for a few minutes before walking up the hill.
There is an entrance fee to get in, which I think was around 30,000 kip. In my opinion, it was absolutely money well spent.