Located northwest of Bangkok in Kanchanaburi Province, the River Kwai (yes, THAT River Kwai) combines some moderately intense adventure opportunities with the chance to visit an intriguing piece of WWII history. Its name was made famous by the Pierre Boulle novel which recounts the history of Allied POWs forced by the Japanese to build two bridges spanning this tributary to the Mae Klong. One of them is still visible to this day, and is a large draw for travelers to the area.
For those seeking to get out on the water, there are plenty of boat/raft trips that can be organized in nearby Kanchanaburi town. The waters are not difficult to manage by any stretch of the imagination, but these journeys are usually part of bigger tour packages that include hiking and the option to ride some (cough cough...often abused) elephants. For those who prefer exploring on two wheels, bikers have their choice of trails that navigate the river and the surrounding countryside, and day trips can easily be arranged to the popular Erawan National Park. Lodging will be easy to find, so don't be afraid to plan a couple days in the area.