The falls of Kouang Si about 29 km south of Luang Prabang are considered the most picturesque in the area. The large multi-stage waterfall, accessible by boat or truck hire.
There are food and touristy stalls outside the waterfalls. It is worth putting a whole day aside (or more) for seeing these because they are a great place just to chill out and meet other travellers. There are multiple pools at different levels, all of which seem save to bathe in, and are extremely picturesque.