Angkor Wat and the temples around Siem Reap are beautiful. My favourite is Ta Prohm with the roots growing up through the ruins, a little bit Indiana Jones.
Built in the late 12th century it is full of history and a guide definitely helps to bring the history alive. They point out so many things you would miss if you tried to do it on your own. On an Avalon River cruise we got to experience the life of the local in the villages along the Mekong River from Cambodia and into Vietnam.