Thoroughly enjoyed our afternoon trip here from Vang Vieng. it take about 45-50 mins by taxi but is well worth it. The lagoon is so cool and clean, lovely and deep too so you can dive in from the overhanging trees.
The caves above are a tough but short hike up a very steep hill. Once inside they are quite vast. What amazed us was that there is absolutely nobody inside to tell you when to stop and turn around, i.e. we didn't know whether you were supposed to keep going and come out at a different entrance or turn back. if we had kept going we would probably be still in there now so be careful! No signs, no guides!
Still, great day out!