I haven't been to any of these places myself, but I would love to go. The Angkor Wat temple in Siem Reap, Cambodia looks amazing. In general, I think there are many fantastic Buddhist temples in these countries that are worth visiting.
When it comes to transport, I think railway services are limited in some parts, so you might have to go by bus. River boats on the Mekong river also looks like an option, though one should be careful with over-crowded boats.