Phnom Kulen (literally meaning mountain of lychees) is more so a mountain range as opposed to a temple site, but it remains a popular tourist attraction when visiting the Siem Reap Province. Rather than a mountain range, it's more so a chain of smaller hilly plateaux that stretch on for over 40km. The range consists of mostly sandstone, and used to serve as a quarry where Angkorians likely mined for the sandstone that makes up so many of the temples in the area. Phnom Kulen is located about 30km North of Angkor Wat, and is considered to be a holy mountain range of religious significance to Hindus and Buddhists alike who come to the mountain in pilgrimage. You could easily spend a whole day here. Between the views of the surrounding bushland and the spiritual sites scattered across the mountain, it proves to be a favourite of tourists travelling around Siem Reap.