Thanks again for the great advice. More questions:

What airline did you use for the multi-trip booking as per quote below?

"We booked Bangkok-Siem Reap-Phnom Penh-Bangkok - booked and paid online. It's worth noting that this airline offers a special fare for a multi-trip journey such as this - good savings - check out the website."

Also, for first time travellers to SEA, travelling out of Bangkok, is it possible to name four or five "must go" places? Our interests and ages vary and some of us are on a spiritual-like-journey.
Thanks.