Si Phan Don, or the 4,000 Islands, is a serene riverine archipelago in the Mekong River in southern Laos, near the Cambodian border. Famous for its laid-back vibe, it features waterfalls like Khone Phapheng and Liphi, rare Irrawaddy dolphins, and relaxed islands like Don Det and Don Khon. It is a top destination for backpacking and nature lovers.