Siem Reap has a number of tourist attractions related to the war that ravaged Cambodia during the end of the 20th Century and the Tuktuk drivers will want to take you to all of them, ideally on the same day!
The Cambodian War Museum was built by Cambodia’s Ministry of National Defense and contains artifacts salvaged when the civil war ended in 1998. It is $5 to walk around it and a free guide is available.
Whilst Angkor Wat is the largest temple around Siem Reap, there are many others some of which in my opinion are better. So you want to take some time and see as many as possible else you’re doing yourself a great disservice.
No matter how you decide to view the temples, you’ll need to buy a temple pass to let you in the area. These passes get checked on the road as you enter the temple complex and additionally by a guard before you enter each temple, do not lose it!