Without a doubt the most popular day trip out of Luang Prabang is the trip up the Mekong River to the caves at the mouth of the Ou river (hence the name 'Pak Ou' - 'mouth of the ou'). Although you can make most of the trip by road, boat is the best way to go even though it does take nearly an hour to make the trip up river
As you head up the Mekong from Luang Prabang, the limestone hills get higher, with sometimes sheer cliffs jutting straight up out of the water. On the bank opposite the point where the Ou River joins the Mekong, a large cave gapes open in the face of one of these cliffs.
The cave has been used for centuries as a repository for old or disfigured Buddha images. Even before Buddhism, the cave was believed to be a place occupied by spirits. You enter cave from the river -- meaning that even if you make the trip by road, you have to use a boat to cross the river.
A path from the main cave (called 'Tham Ting' in Lao) leads around and up the hill to another cave, formerly used as a living place by some of the locals, but now home to even more Buddha images.
A trip to Pak Ou typically includes a stop at the 'whiskey village' about halfway there. The proper name of the village is Ban Xang Hai, but everyone knows it as the whiskey village. Here whiskey is made from fermented rice soaked in water from the Mekong River. You can of course sample the wares as well as purchase a bottle.
If you time it right, starting off in the morning, you'll finish about noon and can have lunch at the village opposite the caves. There are of course several places in the village to satisfy your cravings.