Ang Thong National Marine Park encompasses 42 islands in the Gulf of Thailand, about 45 kilometres southwest of Koh Phangan. The islands are dramatic — steep-sided limestone karsts rising from turquoise water, with sea caves, hidden lagoons, and coral reefs. The centrepiece is the Talay Nai, an emerald inland saltwater lake accessible by steep trail, which the islands are sometimes said to have inspired the novel "The Beach."
Day trips run from Koh Phangan's Thong Sala pier, typically departing at 8am and returning by 6pm. The price (around 1,200–1,800 baht) usually includes snorkelling gear, kayak hire, and lunch on one of the islands. The park is closed during rough weather — mainly from October to December — so check conditions before booking. This is one of the genuinely rewarding excursions available from Koh Phangan and well worth the early start.