The Temple Complex of Angkor Wat was built from the 6th to the 12th century (I also read 9th - 15th) and is steeped in history. There's far too much history to cover in this post, but there's plenty of information on-line about the rich and tumultuous history of the Khmer people and of Cambodia. It's worth researching and visiting if you've never been (Cambodia, in general, and Angkor Wat, for sure). Angkor Wat is probably the most popular tourist destination in Cambodia, from what I gathered, but there's definitely plenty else to see and enjoy. We thoroughly enjoyed our time in Cambodia.
Pete and I had the pleasure of experiencing 5 days at Elephant Nature Park in Cambodia. It was one of the most rewarding and enriching experiences of our lives. Elephant Nature Park is an elephant rescue and rehabilitation center (based in Northern Thailand, with projects in Cambodia & Myanmar as well). Volunteers come from all over to help and visit. They have executed dozens of rescues all over Southeast Asia, allowing for these sentient creatures to live out the rest of their lives in peace and comfort.