One thing is to note that in Siem Reap, you must buy an admission pass to visit the temples and sites in the Angkor Archaeological Park. As you can see in the photo, passes are sold in 1 day, 3 days or 7 days blocks that must be used on consecutive days. One day is only enough just to see the highlights of the most famous temples while seven days pass allows you to explore all the temples and visit many minor structures.
Another necessary thing to have is a passport-sized photo when purchasing the passes although free photo taking is provided. This can take up quite some time during peak hours.
It is a must to always carry your pass around and take care not to lose it as it will be checked upon each park entry and major temples.
A sample of a 1 day pass.