Tonlé Sap has been classed as a UNESCO Biosphere Reserve since 1997 and is a hub of economic activity and nature that is much respected by the Cambodians. It doubles in size during the rainy season, which makes it an interesting place to visit all year round.
You can get to Tonlé Sap from Battambang, Siem Reap, or Phnom Penh, by bus or boat. There are lots of things to do on this lake, whether they are well known or not, and it is an absolute must-see during a trip to Cambodia.
Like most people, you will probably begin your visit with a floating village, which gives you a good idea of what local life is like. If you visit during the dry season, you can admire the 7 metre high bamboo constructions which support the houses when the water level rises. The biggest and most beautiful of the villages are those of Kompong Khleang, which is one and a half hours from Siem Reap , and Kompong Chhnang, which is one hour from Phnom Penh.
Another village worth visiting, near to Battambang, is Prek Toal, which is a Biosphere Reserve because it has such a large number of birds. The reserve is managed by an organisation called Osmose, who have hired local rangers to protect it, especially by preventing the stealing of bird eggs and nestlings. They also organise visits by boat and know a lot about the bird life and diversity of the spot.
In order to make the most of the lake, and for those who love to relax and do nothing, you can also take a cruise from Siem Reap to Phnom Penh. There are some top of the range 4 day cruises, with two excursions each day to floating villages, temples, etc. ... Or for something a little more affordable there also 3 day cruises. If you are lucky you will have an English speaking captain which will make conversation easier.
Finally, if you are going to visit Siem Reap I advise you to also visit the NGO Krousar Thmey which has mounted a very interesting exhibition about Tonlé Sap and its maintenance, and what's at stake for the country's future.