Ignoring Els’ rule to "always do Angkor last on any trip involving Angkor" I opted for one last WHS visit on my way to the airport in Phnom Penh: Sambor Prei Kuk. The site is conveniently located close to the main road between Siem Reap and Phnom Penh. So this was simple stop over, splitting a long bus ride into two more manageable legs.
Sambor Prei Kuk was the first capital of the Khmer Empire and predates Angkor by 300 years. Initially I was worried to get a worse version of Angkor. But the age difference clearly shows in artistry and architecture and sets the site apart. It does have the same jungle setting with trees growing in and on top of buildings. But due to the way smaller crowds (if any) Sambor Prei Kuk is a more intimate experience.
So in summary, the site felt like a nice last visit complementing my Angkor experience. So I would argue that Els original rule needs a revision.
Getting There
The next big town is Kampong Thom. Most buses running between Siem Reap and Phnom Penh will stop here. You can buy tickets at your local hotel or a travel agency.
Via minibus it took 2.5h including pickup at my hotel to get from Siem Reap to Kampong Thom. Travelling on via normal bus to Phnom Phen the journey took 4.5h, 30min spent on a break and at least 1h spent in the terrible traffic of Phnom Penh. I would opt for a minibus if you have the choice.
From Kampong Thom you can take a car (20 USD) or a Tuk Tuk (15 USD) to the site. I opted for the Tuk Tuk and we drove for an hour to get there. The ride was organized by my hotel, but there are plenty of Tuk Tuk drivers roaming the main street in Kampong Thom. Alternatively, you can contact the local tourism board. Finally, Thomas stated that there was also a public bus available, but Tuk Tuk will always be my preferred option if available.
You can do Sambor Prei Kuk easily as a day trip from Siem Reap using public transport. Doing the same from Phnom Penh is probably harder due to traffic.
While You Are There
Kampong Thom itself is uneventful. It boasts some rather simple hotels and restaurants.
Main nearby attraction would be Tonle Sap Lake, the largest fresh water lake of South Eastern Asia. The lake is unique as the flow to the Mekong reverses during high tides. It is already a Unesco Biosphere Reserve, but maybe it also has potential for a world heritage site.