One area of Angkor I have been fascinated by but never able to visit until now is Kbal Spean. While not a secret, it is still more of a “behind the curtain” experience than the rest of Angkor. It is quite far out from the main sights and requires an uphill hike of about a half hour to reach it. 
While created by the Khmer empire like the rest of Angkor, Kbal Spean is completely different from the rest of the temples one visits. The sight boasts carvings in a riverbank and hundreds of stone lingas in a stream. Sometimes called the river of 1000 Lingas, this is not a spectacular sight like Angkor Wat or any of the major temples at Angkor. It is more subtle and personal - definitely carved on a human scale. It is located in a hilly area from which the Khmers quarried the stones needed to build the great temples of Angkor.
I had polled my group before the tour to see if there was any interest in a visit to Kbal Spean. About a dozen intrepid tour members opted to accompany me out to the sight. I thought it was really fantastic.
The travel distance required to get there keeps the destination from being overwhelmed with tourists. Even so, there is a decent infrastructure at the parking lot. A new, clean restroom facility, plenty of parking spaces, and about a half dozen basic restaurant/food stalls are one's first experience upon arrival. We grabbed a quick lunch and some cold bottles of water before starting up the trail.
The trail itself is not steep but the steady uphill grade, coupled with the tropical heat, makes for a demanding trek to the sight – another factor in keeping visitor numbers down. I wouldn't describe the hike as scenic, but you are making your way through true jungle terrain. Vines occasionally hang over the path and 
gnarled root systems erupt everywhere on the trail. About 3/4 way up there is a beautiful overlook where you see a vast jungle valley stretching out for miles. At a couple of the steepest sections wooden steps have been built but there are still several sections that require some careful stepping up and over stones and roots. One annoying feature, at least for me, was a series of markers, denoting how far you have come and how far you have to go - in intervals of 100 meters. I for one would rather not know how far I have to go, at least not so often.
The river and the carvings themselves are not dramatic but I found them completely enjoyable and well worth the effort to get to them. The carvings are spread out over several hundred feet of terrain and can be easily missed. The security guards assigned to the area are happy to serve as personal guides - making sure you see all the carvings. They expect a tip of course, but I felt no overt pressure to give more than seemed fair and I thought it well worth the small investment. Among the carvings I saw were several mandalas, various creatures, Shiva, and Vishnu, plus hundreds of lingas in different locations. There is supposed to be a frog somewhere but I did not find it. My favorite stop was the small but picturesque
waterfall. About 20 feet high, the small cataract makes for a cool and refreshing picnic spot. There were locals there doing just that during my visit. The water flow was very low and I can see how more water would make everything more dramatic. Going down was much quicker but one needs to tread carefully on the rutted, root-riddled path. Once again the food stands at the bottom were welcome as we all needed more water upon arrival. Overall, it was a great added excursion, and due to its uniqueness, I thought it well worthwhile.