Cambodia uses a mix of US dollars and Cambodian riel. Any change under $1 is given in riel. US coins are not used.
If you're coming from Bangkok, fly with Air Asia or Bangkok Air. Bangkok Air will be a little more expensive but the service is superb. They have a separate lounge at BKK with complimentary food and drinks. If you fly with Air Asia, you will need to depart from DMK, just a heads up.
Coming from Ho Chi Minh, there's only one airline, Cambodia Angkor Air. Tickets are expensive at $135 but it beats a long brutal bus ride. A 30-day visa can be applied for on arrival for Americans (check current regulations for your nationality). Make sure to have a passport photo or you'll get fined.
Funky Flashpacker - For a nice social environment, stay at Funky Flashpacker. They have a great pool and rooftop bar. Music is played all day around the pool and then the party moves up to the skybar in the evening. It's easy to meet people here if you're a solo traveler.
Topsky Hostel - When you're ready to get some sleep, stay at Topsky Hostel, just around the corner. Talk to the owner Manh, who is a great guy and trustworthy, and tell him I sent you. If you need to work out, ask him to bring you to the local gym with him.
My tip is to stay at Topsky Hostel since the private rooms with AC are less expensive than at Funky and you can still get access to the facilities at Funky since the owners have a mutual agreement.
Here's a list of my favorite restaurants and cafes in Siem Reap.
Sala Bai - An establishment that trains locals in the hospitality industry, reducing the risks of poverty and human trafficking. Set lunch meals are a great value and meticulously presented.
Chae Ngek, 9999, Siv Long, My Little Cafe - These are a few of the numerous cheap street eateries down the street from Topsky Hostel near the corner of Samdech Tep Vong St and Taphul Road for those on a super tight budget. Rice dishes start at $1.
Angkor Cinema - Located in the Angkor trade center next to the old market. You can rent out private rooms with your friends and screen movies for $5.
Angkor What? - A bar with a good happy hour which turns into a dance club later on in the night.
Pub Street - This street has turned into the spot for partying. Touristy but fun in the evenings.
Psar Chaa (Old Market) - Wander through the stalls for some cheap shopping.
Phare Cambodian Circus - Like Cirque de Soleil but carries a subtle social message. $18 Adults.
Leave at 4:30am so you catch the sunrise over Angkor Wat, it's absolutely amazing. I rank it just a few spots under Machu Picchu. As soon as you catch the sunrise, run around to the back side of Wat to get a few minutes to take photos without the crowds. Then make your way over to Ta Prohm (where Tomb Raider was filmed) which was my favorite temple. The jungle and tree roots intertwine with the walls of the temple and it's quite a spectacular sight.