Tat Kuang Si park is a wonderful spot for a hike.
The water has an amazing blue color that seems almost fake in photos but is really the color of these photos. The blue is due to dissolved copper.
The Kuang Si waterfalls are about 30 km outside of Luang Prabang. You can rent a open back pickup for about $30 (really I forget I think it was about that amount though) for the trip or you can join a tour group (easy to arrange at many guest houses and travel agents all over Luang Prabang).