The Luang Prabang Tat Kuang Si waterfalls are amazing and a great half day trip.
The water is beautifully coloured and get for swimming, the forest cool and peaceful, and there’s good cafe (average price, good food) for lunch that overlooks the falls.You can also take your own picnic or buy food from the food stalls at the entrance to the falls.
The chicken in red curry with sticky rice is wonderful (and better than the fish in red curry).
For around $8 AUS (50,000 kip), you can book a seat in a minivan to travel by road ( around 40 mins each way). But the trip may well be best by boat (2hrs one way+ 1hr the other (due to river flow) 40,000 kip including 10 min tuk tuk ride each end) because the minivan is commonly full (even overbooked) making it pretty unpleasant if you end up in the back seat.
Once there, take the path that winds along the falls up to see the long drop at the top (a short, cool walk), then come back down to the picnic tables/cafe and swimming spots. Chnging rooms are provided.
(If you are keen on being the river by boat it is popular to travel from Luang Prabang to Northern Thailand (approx 250,000 kip per person including overnight at Pak Beng, bookable in Luang Prabang through travel agents like Green Discovery)).