Bayon Temple
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Located in Angkor Thom which is a short distance away from Angkor Wat, this city is
well known as the "Intersection between heaven and Earth". Bayon is known for its huge stone faces of the bodhisattva Avalokiteshvara, with one facing outward and keeping watch at each compass point.  There are 51 smaller towers surrounding Bayon, each with four faces of its own. If you are visiting Angkor Wat, do give Bayon Temple a visit too!
Angkor Wat

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Angkor Wat, one of UNESCO's World Heritage Site is probably the most well-known
historical place in Cambodia.
Filled with history and wonderful architecture, one could walk for an hour and so exploring intricate details of the city itself which was built to honor the Hindu God, Vishnu.

You can pay a total price of 50USD to visit the three temples in this area, or 25USD per temple.
Angkor National Museum
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Despite the criticisms in year 2007, The Angkor National Musuem took it as a sign for change and has done so during the years ahead. Now, the only Museum of Khmer Sculpture in Siem Reap has been garnering quite a number of visitors who are curious to know more about Angkor and the history of the state itself.

The Museum itself does play an important role for people who are seeking for knowledge but there is nothing more than that. If you are not a history or cultural interested junkie, we suggest you steer clear away from the Museum.
The entrance price is set to 15USD though you can get it cheaper if booked earlier.
        
Kampong Phluk
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Roughly an hour ride away from Siem Reap, Kampong Phluk is actually a village on sea
which is supported by stilts. Instead of cars, each house will have a wooden boat parked infront.
A boat ride here will last roughly 45 minutes and you will be taken around the vicinity of the village.
Entrance fee can cost as low from a dollar to forty, depending on your luck.

Koulen Mountain

Roughly an hour and a half drive, Koulen Mountain is located in the outskirts of Siem Reap and away from the bustling city.
For roughly 45-60USD one could get to this waterfall and take a dive.
Usually locals will just stop you at the top of the waterfall which is less impressive than the one located below. The bottom of the waterfall is accessible by a flight of stairs behind some trees. Make sure to descend to the lower waterfall to get an amazing view.

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Fancy a little drink or a place to wind down and shop, then Pub street should probably be on your list. Located right smack in the middle of hotels and malls, this market has everything from clubs, pubs and cheap stalls which sell souvenirs and more.
Apsara Dance

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A traditional dance which was only practiced in royal palaces can now be seen practically everywhere in Siem Reap.

The Apsara dance differs depending on the location. There are free shows around pub street, and you can enjoy your drink while watching, or you could go to some theatres and watch their performances along with some others like shadow puppet stories and such.

Phnom Bakheng

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Located 1.3Km away from Angkor Wat, the site famous for it's sunset and sunrise view stands proudly overlooking other temples and sites.

Be sure to put this place in your list if you enjoy watching a great view.
Photographers and enthusiast alike often flock to this area during dawn to catch the sunrise and, personally I do have to admit that the view is amazing!