Huay Xai (also spelled Houeixay or Houei Sai) sits on the Mekong in the far northwest of Laos, directly opposite Chiang Khong in Thailand. For most travelers, it serves one purpose: as the starting point for the two-day slow boat journey down the Mekong to Luang Prabang, one of Southeast Asia's classic river trips.
The slow boats leave each morning and take two days, overnighting in the riverside town of Pak Beng. The scenery is wonderful — limestone cliffs, dense jungle, and small villages along the riverbanks. Bring snacks, a cushion, and something to read.
The Gibbon Experience, a zipline and treehouse eco-adventure in the Bokeo Nature Reserve, is based near Huay Xai and is one of the more unusual things you can do in Laos. You sleep in treehouses high in the forest canopy and travel between them on ziplines, with a chance of spotting endangered black-crested gibbons. Book well in advance.
The town itself is small and pleasant enough for a day. The temple of Vat Chom Khao Manilat sits on a hill above town with views over the Mekong. The riverside strip of guesthouses and restaurants is a good place to spend an evening waiting for the slow boat. The Laos-China Railway has a station at Boten, about 200 kilometres to the northeast, which has opened an alternative route into northern Laos from Yunnan province.