What is the best way to get from Chiang Mai to Chanthaburi, and then from Chanthaburi to Bangkok. I've checked the train schedule and it doesn't seem to go to Chanthaburi.

The fastest and easiest way would probably be to connect via Bangkok by bus. Lots of options if you go this route...or you could take the train to Bangkok, but you'd have to connect with one of the bus stations to get to Chanthaburi. Ekamai or Morchit. Plenty of options there.

i agree with craigT. Get down to bkk first and take care of a ride from there unless you are willing to spend bigbath. A few months ago i shared a (dodgy)taxi ride to chanta from bkk one way at B500 a piece. Did the buddah footprint thing, walked the mountain, etc. Kao kitchakut it' s called if memory serves. Anyways it' s at Chantaburi.