do you want to see the Angkor Wat, the world largest temple in the world ?
do you want to see lot of great temples built by the great kings even the king of Egypt cant effort with secret and complicated plan?
do you want to know how the Ancient Empire started war or made war to control most of those part in Southeast Asia(Khmer Empire)
if you want to know all these, The best place to visit is Angkor wat temple in Siem Reap. and the beach in Shiahanouk Ville, very natural beach, alot of places to visit but it depend on the time you have, if you want to see the VERY natural mountain or forest it takes you so much time but i can recommend you somewhere like Pailin in Battambang Province(waterfall, Blue Mountain ....), Rattanakiri and Modulkir (waterfall, natural mountain, wildlife....) very very green country.
acutally there is no difficult rule for you to be aware of. The big thing is you have to aware of is accused of being sex with children.
The Hotel rate is cheap, you can find it every where when you reach Phnom Penh, guest house is more cheaper.
everything here is not expensive at all.
people are very nice and their english not bad.