The sprawling archaeological ruins of Koh Ker are a rewarding day trip from Siem Reap due to the number of ruins, the size of the site, and the fact that few tourists visit. I've often found myself alone at temples, or at most with just two other people and a guide. While the Koh Ker temples can be visited any time of the year, they're more alluring during the rainy season. Located in a savannah landscape the setting can look very dry in the 'winter' months of December to February. Come May when it starts to rain, the vegetation takes on an almost iridescent green, moss begins to dapple the temples, and flowers flourish in the surrounding forest.
Tips: wear a raincoat, hat, sturdy waterproof shoes (things can get slippery and muddy) and take an umbrella and waterproof bag for your camera and valuables. Leave in the darkness so you arrive at the crack of dawn. When you're hungry, head for the food stalls. That fragrant stir-fried meat they're suspiciously serving up? No, it's not dog as many foreigners suspect. I was told it was beef, however, it's actually incredibly delicious venison. I later discovered that the deer is endangered. Hence the hesitation from staff if you say "I'll have what he's having."