When you are crossing the border from northern Thailand (Chiang Khong village) to Laos (Houeisay village), you can take 2 days slow boat trip on Mekong river to Luang Prabang with 1 night over stay in Pak Beng village.
In high season, slow boats operates every day in morning, leaving between 10am – 11am.
Port for this slow boats is located here, GPS: 20.284907, 100.406881. You can buy a ticket directly from the Ticket office, but only in the morning from 8am till 11am.
My photos from slow boat trip on Mekong river from Houeisay village to Luang Prabang town with 1 night in Pak Beng village: