With the benefit of hindsight, I can tell that my lethargy from a couple of days ago was probably due to heat exhaustion. I'd spent several hours wandering around Angkor Wat in the middle of the day and was drenched in sweat. A couple of days resting in an air-conditioned hotel room has restored me. Yesterday I visited Angkor Thom, another bunch of old ruins near Wat, and they were great. One called Bayon was particularly fun with lots of crazy passageways to get lost in.
This evening I had a sunset dinner on Tonle Sap Lake where there is a floating village. This was great, contrary to my lonely planet travel book which dismissed it as 'too popular'. There were other tourists but not too many and it was interesting to see how people live on floating huts which they move up and down river depending on the season.