There are a number of spots near Kanchanaburi, over and above the 'usual' touristy spots. Around ANZAC Day (=memorial day for those from US) on 25 Apr visit Hell fire pass museum( the flame trees are in full blossom by then), Home Phu Toey Resort (includes some direct history with owning family and resistance against the Japanese), puengwaan resort (Sai Yoke) and others. In 1986 I visited Doi Ahng Khan, near Fang, on the Thai Brunese border with some student from Thamasat University. Was quite a trip with a Royal agricultural project running to encourage locals to depart from opium cropping..... Dunno how it has gone.

Beaches, Well if you have done the main places try near Cambodia, with Ko Chang, Ko Kood and others. Quickly into the region where infrastructure can only support aircon 1800-0600 with itinerant power, and no Internet (well, decent connectivity anyways).

Forays into Cambodia highly recommended, but so much depends on what you are prepared to 'tolerate'!

Cheers, TK