The war-history route a day in town
Start as early as you can. For the in-town route, take the train from Thonburi station (atmospheric, with scenery before you arrive) or a minivan from Mo Chit 2 / Sai Tai Mai (quicker and more frequent, ~2–2.5 hours), dropping you near the cemetery and the museums. By train you watch the western countryside slowly give way to river and hills along the way.
Begin at the Don Rak War Cemetery in the town centre — the immaculately kept Commonwealth war cemetery with around 6,982 graves of Allied prisoners of war who died building the Death Railway. It is a place to walk quietly and with respect. Directly opposite is the Thailand–Burma Railway Centre, a thorough, serious museum that tells the story of the railway's construction, its background, and the lives of the prisoners — visit it before the bridge so the history makes sense.
The town also has the JEATH Museum by the river, which recreates a POW camp, and across the river the quieter Chong-Kai cemetery, both worth a stop if time allows. But for a single day, keep the focus on Don Rak and the Thailand–Burma Railway Centre.
Walk on to the Bridge over the River Kwai — the black iron railway bridge that has become the image of Kanchanaburi. It was built by prisoners of war and Asian labourers on the Death Railway, bombed and rebuilt; the curved spans are the original ones, the angular straight spans the post-war repairs. You can walk across it (there are refuge bays to step into when a train passes), and the local train still crosses it. Walking across is free.
From late November to early December there is the River Kwai Bridge Week, with a light-and-sound show that tells the history — if your trip lands in that window the atmosphere is especially busy. Whenever you go, keep the bridge's origins in mind and visit with respect.
Take a break for a properly local Kanchanaburi lunch — freshwater river fish at a floating raft restaurant. Around the bridge and along the river there are several floating raft restaurants where you can eat yi-sok, snakehead or freshwater catfish, grilled, fried or in a tom yum, with a view of the river and hills. It is a flavour you rarely get in Bangkok. See more places and dishes in the Kanchanaburi food guide.
If the train timetable lines up, this is the highlight worth doing — ride the Death Railway from the in-town / River Kwai Bridge station toward Tham Krasae. The train clings to the cliff on the Tham Krasae wooden trestle (the Wampo viaduct) above the river — a rail view that is beautiful and sobering at once, because this stretch was built at the cost of many prisoners' lives. There is a cave shrine (Tham Krasae) beside the track, and the photo spot for the wooden viaduct is right there.
One thing to know: train services are limited and the timetable can change. If you ride to Tham Krasae in the afternoon, check that there is a return service back to town in time to connect with your trip back to Bangkok — otherwise keep a minivan or private car as a backup. If the timing does not work in a single day, save the train for an overnight trip, which is more relaxed.
If you came by minivan or private car, leave town around 16:00–17:00 to reach Bangkok before nightfall, allowing ~2–2.5 hours plus traffic on the way into the city. Evening minivans run frequently, while the return train services are limited and slower — so if you are banking on the train back, check the timetable in the morning and have a backup.