Where the Nam Ou (Ou River) and Mekong River meet at Ban Pak Ou, two famous caves in the limestone cliff are crammed with myriad Buddha images. In the lower cave a photogenic group of Buddhas is silhouetted against the stunning riverine backdrop. The upper cave is a five-minute climb up steps (you'll need a torch), 50m into the rock face. Buy boat tickets from the longboat office in Luang Prabang.
Most visitors en route to Pak Ou stop at the 'Lao Lao Village' Ban Xang Hay, famous for its whisky. The narrow footpath-streets behind the very attractive (if mostly new) wat are also full of weavers' looms, colourful fabric stalls and a few stills producing the wide range of liquors sold.
An alternative is to go by road to Ban Pak Ou (30km, around 150,000K return for a tuk-tuk), then take a motor-canoe across the river (20,000K return). Ban Pak Ou is 10km down a decent unpaved road that turns off Rte 13 near Km 405.