My first choices for hiking/ back packing are Laos and Vietnam. In Vietnam, as Nomade suggests, there is some great hiking in the far north. In addition to Ha Giang, I also loved the Mai Chau valley, the hills around Dien Bien Phu, the Ba Be lakes. Sapa also has some great hike out into the surrounding villages although the town does get very busy.
In Laos, the areas around Phongsali and Muang Sing in the far north have some great trails through the forested mountains and the minority people's villages. In the south the Bolavens plateau is very beautiful and although there are no beaches, the area around Si Phan Don ( 4000 island) is a great place to chill out on the Mekong .
Thailand is much more developed and I doubt would provide the sort of experience you seem to be looking for. I really like Cambodia but would again agree with nomad es comment re the mountains and beaches
while cambodia is developing, it is way behind the other 2. siem reap, the locaation of the angkor war ruins is the country's primary sight...