I'd definitely recommend going through your hotel or a local company like Loca to arrange transport - it helps support the local economy. As for waterfalls, we really enjoyed Kuang Si for its lush jungle setting. The Living Land farm was a great way to learn about Lao agriculture too - just ask plenty of questions. Most of all, don't rush your time in Luang Prabang. Be sure to wander the quiet backstreets and soak up the laidback atmosphere.