The city and province of Kanchanaburi, west of Bangkok, may be the first destination in Thailand where tourism got a big boost from a Hollywood movie. The 1957 David Lean film "The Bridge on the River Kwai" had people seeking out the 'real' bridge featured in the movie. The problem is, the film, like the novel it's based on, is pure fiction. There really wasn't ever a bridge over the River Kwai.
However, the Thai-Burma railway (commonly known as the 'Death Railway') built by the Japanese using prisoner and slave labor during World War II, is real, and the memorials to the many British, Australian and Dutch prisoners of war who died building it are still to be found in and around Kanchanaburi town (see cemeteries and museums).
Aside from the WWII and tenuous film connection, the main attraction Kanchanaburi these days is nature and adventure, with activities such as rafting, trekking and exploring caves.
Like most of Thailand, Kanchanaburi is best visited during the cool dry season from November to February (see weather). The rainy season can be a good time to visit, when there are fewer people and the scenery is lush and green, but it seems that many small tourist businesses close for the season, from May or June until September or October. However, hotels that do stay open all year usually offer quite attractive rates for the 'green' season.