Concierge.com's insider take:
Just south of Siem Reap lies the Tonle Sap ("Great Lake" in Khmer), the largest freshwater lake in Southeast Asia. One of the hydrological wonders of the world, the lake quintuples in size during the summer, when the monsoon-swollen Mekong rises so sharply that part of the torrent veers into the Tonle Sap River at Phnom Penh, actually reversing the current of this 60-mile waterway and raising water levels of the lake more than 25 feet. To handle the annual fluctuation, fishermen live in stilted houses or floating villages, complete with floating schools, police posts, temples, and crocodile pens. Local nonprofit Osmose (12-832-812; www.osmosetonlesap.net; firstname.lastname@example.org) offers day trips and overnight stays in Prek Toal, a large floating village, and bird-watching in its nearby UN Biosphere Reserve, a sanctuary for such rare species as the greater adjutant (a bird, not a military officer).