Spending a night in Kanchanaburi unlocks a different experience — river floating bungalows, WWII trails, and Erawan waterfall.
Death Railway Bridge, Thailand-Burma Railway Centre (museum), Allied War Cemetery — done in half a day; don't skip the museum.
Seven-tiered waterfall in Erawan National Park, 80 km from Kanchanaburi town — turquoise water at each pool, small fish that nibble feet; only reachable if you have a day.
Bamboo floating bungalows on the river — basic but atmospheric; book via guesthouses in Kanchanaburi town; about 400–800 THB/night.
An 18km section of the Death Railway cut through solid rock by Allied POWs — the Hellfire Pass Memorial Museum (free, Australian-maintained) is excellent.
Trains and minivans run from Bangkok's Thonburi and Bang Sue stations respectively, roughly 2–3 hours each way — a private driver for two days gives the most flexibility across the spread-out sites.
Day one for the bridge, museum, and cemetery plus an overnight raft or riverside stay; day two for Erawan Falls or Hellfire Pass before heading back to Bangkok in the afternoon.