i went same time as you a few years back its just at the end of moonsoon season which is quite an experience. like others said lonley panet guild and others out there will tell you where to go.
As said Koh chang is a must i think most beautiful island there imo i stayed at tree house, lonely beach. Watch out for needles if you go to full moon party do not take your flip flops off on the beach, i wore trainers just in case, saw two people with needles hanging out there foot, NOT GOOD.
In phnom phen go to a place called lakeside there are lots of guest house cafes and bars Drunken frog bar is a must and so is the lazy gecko cafe ask for an aussie burger, i gaurentee you will not eat a tastier burger in the world FACT. make sure you visit S21 prison and the killing fields.
when you leave take your passport, docs, cashpoint card (nationwide) and the shirt on your back you need nothing else is the best way to it buy everything when you get there. i spent loads of time before going buying all the essentials and worrying about what to take then ended up carrying around a load of stuff that i didnt. they have tesco there too oh and 7eleven on every corner.
hope this is of use to you.
p.s. ping pong show is a must