Battambang is a good place to get to know a moto driver and do some day trips into the countryside.
kratie is small in a really nice way. see the dolphins and bike around.
ban lung was a really enjoyable place to chat with people in the market, relax, and enjoy nature. it's not hard to get away from the crazy touristy side of cambodia as it is pretty centered on siem reap and phnom penh (i still enjoyed both), and i hear sihanoukville, though we didn't go there.
it's a great country to spend some time in and not many people give it more than a week.
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We spent a month in Cambodia this June/July my recomendations apart for the obvious Phnom Penh and Siem Reap would be:
Kampot, Kep and Rabbit Island. Check to see if Bokor is open, spend a day on Rabbit Island in the peace and quiet. Enjoy the rivers and caves of Kampot and of course the food and laid back- ness of the place.
Battambang - do a cooking course at the smoking pot, ride the bamboo train, enjoy the great food! Hire a Tuk Tuk and get out in the country and villages.
Kratie - Irawaddy Dolphins and once again the laid back ness of this town along the banks of the mighty Mekong. Hire bikes and ride around the island across form the township.