Kep

Kep used to be the playground for the rich in Cambodia as it was near the capital. But after the wars it has largely gone into disrepair. Still there are many grand old colonial houses overlooking the bay that bring to mind how bustling it must have been during it's hay day.
Last I visited a few years ago it was really quiet and peaceful and you could get accommodation for just a few dollars overlooking the ocean.
Also cheap seafood in the stalls lining the main boulavard.
If you do spend some time here I recommend a trip up to Bokor National park. Theres an old township up on the mountain. The road is not so good but it is very atmospheric. There's an old church and also an old casino which overlooks the sea. Imagine about 800m above the sea level with stupendous views into the gulf.
Theres also a waterfall up on the mountain.