The easy way to do the trip is to hire a car (& driver if you don't fancy challenging all the Thai drivers
)
Or you could take a morning bus to Bangkok, make your laborious way through the city and catch the direct train from THONBURI Station to Kanchanaburi, Alternatively, make your laborious way across Bangkok to the Southern Bus Terminus and get a bus to Kanchanaburi. Bus is considerably faster than the train.
Even going by car, I'd recommend staying at least one night so you have plenty of time to do everything you want to. As said above, a visit to Hellfire Pass & the museum there is well worth doing and makes a good day out from Kanchanaburi.
Enjoy