The nearest cave temple to Kanchanaburi town is a fun labyrinth of illuminated passageways. The marked trail is a bit of a squeeze in some places (and can be slippery), but ducking beneath limestone protrusions to discover these subterranean shrines is a special experience. It's a stop on the standard boat tour, but still rarely busy, so it might just be you and some fluttering bats. The temple is 4km southwest of town, beyond Chungkai War Cemetery.
The chambers painted white near the exit were a Japanese wartime hospital, and outside the exit, in a replica bamboo barracks, is a little display of POW photos and old typewriters. Follow the roads to the back and up to the stupa for a lovely view over Mae Nam Khwae Noi.