Vang Vieng could be considered a touristy town, but don't hold that against it. It's a magical place, and if you don't find it floating down the river, rent a bicycle and head straight out into the countryside.
This image was taken after climbing down into a cave to swim in an underground lake, and just before watching a river of bats fly overhead on their way to dinner. Every moment in this place felt like a fairy tale, and the landscape only made it all the more surreal.
Bicycles can be rented from any hostel in town, but make sure to come prepared with a flashlight. On a side note, don't pass up the barbecued fish being cooked on restaurant patios all over town, it's some of the best I've ever had.