Great post, fascinating country!
I can recommend to check out the island "Koh Rong Sanloem", it's a remote paradise.
Another place to get to know more about the tragic khmer rouge history are the Killing Fields near Phnom Penh - the audio guided tour is really interesting, shocking and worth the price.
In Kratie you can find a nice riverside walk, many delicious meals, and more..definitely try the "Beef Lok Lak".
Also Kampong Cham is an amazing place, there is the longest bamboo bridge, which you can also cross by car/scooter.
Cheers!