I recommend visiting - Kanchanaburi & The River Kwai, Thailand
As a war history buff, I'd recommend adding Kanchanaburi to your itinerary. Located only three hours outside of Bangkok, Kanchanaburi is the historical site of the Burma Railway (aka Death Railway) that was built by World War II Japanese prisoners of war, as well as the infamous Bridge on the River Kwai and Hellfire Pass. We arrived to the bridge aboard a long boat and took a local 90-minute train ride to Hellfire Pass. Along the route we visited World War II museums and trekked a short portion of the route through Hellfire Pass.