I read many posts on this blog about what to do in Luang Prabang.
I would like to suggest that tourists to LPB go to one of the many Buddhist temples and spend some time talking with the Novice Monks, then do something to improve their lives. These boys come from impoverished villages--many are orphans, and all are poor--and connecting with them will change both your lives. I sponsored a Novice Monk three years ago, and that friendship has exploded into a universe of kindness.
It costs me the equivalent of the cost of a daily cup of coffee to support Onsi at University, and I have received in return not just excellent Buddhist karma but one of the finest friendships of my life. It's soooooooooo easy.
In Luang Prabang I watch rich tourists staying in upscale hotels with their fancy cameras stuck in the faces of Novice Monks gathering alms at 6:00 am, and they leave nothing behind, but take their photographs home. The $100 spent in one night at some fake restaurant could pay for a year at the Mekong English School for a Novice Monk. Backpackers, in my experience, are the most generous of all tourists to Lao people, and the richest of tourists leave the least behind.
Luang Prabang is a magical place. It is filled with Buddhism, with good Lao people, but these are poor people too, so please make a connection when you are there. Change a life or two and watch what happens.