The slow boat journey from Huay Xai to Luang Prabang takes two days and is one of the classic Southeast Asian trips. Boats leave from the pier below town each morning and travel downstream on the Mekong, passing limestone cliffs, jungle-covered hills, and small villages along both banks. The first night is spent at Pak Beng, roughly halfway; the second day brings you into Luang Prabang in the late afternoon.
Bring snacks, something to sit on beyond the hard wooden benches, and something to read — the journey is beautiful but long. The boats can be crowded in high season. Earplugs help with the engine noise. Buy tickets the afternoon before departure from the ticket office near the pier; don't buy from guesthouse touts, who charge a commission.