This page basically covers a long day trip we took from Siem Reap. Our primary target was the temple at Preah Vihear so that was our first stop. It's three hours by car from Siem Reap (with a breakfast stop in Anlong Veng). The temple is in the Dangrek Mountains along the Thai-Cambodian border.
You have to park at the bottom of the mountain and take a four wheel drive truck up to the top of the mountain. This is partially due to the conflict between Thailand and Cambodia over access to the site. At times that has been a violent conflict but in recent years all has been quiet. There is a very noticeable Cambodian army encampment on the mountain. They are there watching the Thai army encampment one mountain over. Nothing about it seemed particularly tense during our visit.
Preah Vihear was originally a Hindu temple built in the 10th century. It has been added to incrementally over the years and throughout the Angkor period. It is basically linear starting at the stairs in the first picture and aiming up the mountain through a series of terraces, long walkways flanked by naga and the occasional temple.
This is the 'end' of Preah Vihear where you run out of mountain and there's just an enormous cliff looking out over Thailand and Cambodia. It was also raining when we visited so the visibility was limited.
When we could see, there are great views of the Dangrek Mountains. There are several impromptu Buddhist shrines located along the cliff edge as well. When we visited there was no museum on the site, or at the parking area. There is a minimal map in the area where the truck drops you off.
We spent much of the day driving around sparsely populated parts of Cambodia on lightly trafficked roads. Much of this part of Cambodia is just open land, not used (obviously) in agriculture. There are some small villages of army housing which did not generally appear to be used. In fertile areas there are plantations of banana, coconut, sugarcane and peppercorns like these.
Our second stop was Koh Ker - a huge archeological site in the jungle 120 km northeast of Siem Reap. There are many many ruins inside the site boundaries but it has not been fully cleared of landmines so it's best to stick to the main areas or travel with a guide. The highlight of Koh Ker is this seven tier pyramid or 'prang'.
There is one road into Koh Ker with parking areas at different ruins along the way. It's remarkably empty if you've been at Angkor. The main area is in front of this pyramid where there are restaurants and market stalls. Just a little further down the road are shockingly nice restrooms. This pyramid is reached through the ruins of an entrance pavillion decorated with Sanskrit writing. You can climb to the top as long as you're willing to brave these stairs.
At the top you'll be able to mostly see jungle in all directions. Far out in the distance are the mountains along the Thai border (including Preah Vihear). If you're lucky, there will also be a Buddhist monk.
As picturesque as the pyramid of Koh Ker is, it may be surpassed by Prasat Prahm which is one of the first areas you'll pass when entering the park. There are five towers here (prahm means 'five') and they are in various stages of being devoured by giant fig trees.
We saw a lot of ruins covered by fig trees between Angkor, Beng Mealea and here at Koh Ker. These might just be the most impressive of the bunch.
Beng Mealea was our third and final stop. We just made it here before it 'closed' for the evening. Closed is a loose definition in Cambodia. Beng Mealea is only 40 km from Siem Reap and can be visited easily as a day trip so it isn't as remote as Preah Vihear and Koh Ker. In general the temples are in ruins but there are some nice fragments here and there like this naga head.
The Khmer Rouge used the temple as a base of operations during the Cambodian civil war. It has some underground tunnels connecting different portions of the buildings and they are open to the public (not sure if we would've found them or not without a guide). Everything is moss covered and it's all rather photogenic.
Because of the state of the ruins, the pathway through the interior of this temple is elevated and you don't have as much freedom to wander. There are some nice viewing platforms though. We visited just before sunset and it was nearly empty.