I went with San Park to Phnom Kulen waterfalls a few days ago, which is about an hour/hour and a half drive from Siem Reap. the views to it alone were gorgeous enough, but "oh my buddha!" (as San Park always says) the waterfall is BEAUTIFUL. I think the people who have negative views only saw one- the smaller one. there are two. the small one has the carvings in it, and the bigger one is a little trek down from there. and the bigger one is STUNNING, and as it is considered to be holy water and to bring about "prosperity", we all took a long dip in it, which was fabulous in the crazy midday heat! you can borrow swimsuits, so don't worry if you don't have one. an absoloute MUST!! and a nice change from looking at the temples (even though they are mind-blowing!).
PS: the reviews of San Park are all true- he makes any trip 30000 times better! make sure you catch him before you leave your country. (firstname.lastname@example.org) his rates are really good, he is a super-knowledgeable, punctual, friendly, and super funny guy! I would recommend him wholeheartedly to anyone who is thinking of visiting Siem Reap.