I've been backpacking there for 8 month last year... Trust me you'll be fine, and it's a life changing experience

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Just watch for your stuff but in all honesty, I had absolutly no problems during my trips. Asians people are the most open minded, and the nicest I have ever met. So, really, don't be afraid to go talk to them, ask question, introduce yourself and you'll meet some quiet intersting people

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For Cambodia, take at least 2 or 3 days to visit Angkor Temples, it's worth your time. Rent a mortbike driver or rent a bike if you're motivated. I really liked Phnom Penh but you'll see by yourself. I've been to all south east asia but it was in Phnom Penh that I got the most harrasment with the kids asking you for stuff or trying to sell you some books or so. It's a bit annoying but just say no and ignore and you'll be fine. If you like to go out, there are plenty of nice bars and club in the capital or in Sihanoukville, but watch out for the hookers.
Eat local too, so many great things to try.. If you take the bus between Siamreap and Phnom Penh, it will stop in between for lunch in a small market bus stop type of thing. BE SURE to try the local specialty: fried tarantulas

! There are plenty of them, just buy one, and taste a leg

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You could rent a motorbike and travel a bit around, off the tourist trail and get lost, it's the best way to really discover Cambodia, thats what I've done. You'll get some flat tires but don't worry there will always be someone who will help and usually for nothing.
If you want some more infos, on cambodia (hotels, ghuest houses, and stuff) or for any other countries around it, tell me I'll be glad to help.
For info I've been to:
Indonesia (sumatra, java, bali, sulawesi) , Malaysia, Borneo, Thailand, Myanmar, Laos, Cambodia, Vietnam and China.
Cheers !