Went by minivan from Siem Reap to Stung Treng. This little "Mekong-Town" has a nice market with lots of street food. The Road to the border is hard to ride, very dusty and nasty ripped. In Laos the road conditions are very good. 10 km behind the border are the bombastic Mekong Falls. The little village Nakasong has the ferries to get to Don Det Island which has even hotels with pool area. All tourists there with rented trash bikes watched me envious on my bike as the dirt tracks are very bad.
Base was Koh Kong. Starting 50km on concrete road, then it became very bad. No refreshments to buy there, so I drank water from a creek as there is no population at all. Got no problems from that. The area is formed by banana plantatations and other crops agriculture, even a big pepper farm is along the way. Osoam Village is close to a giant artificial lake. The water is clear and the atmosphere is amazing.
Starting by minibus from Sihanoukville to Kampot. Crossing vietnamese Border near Ha Tien towards Ferry terminal on the other side of Song Giang Thanh River. Speed ferry takes 75 minutes. Accomondation in Duong Dong is easy to find. The Cable car in An Thoi is expensive but unbelievable exciting, a must!!!
My plan was to travel more, but at this time of season around noon and early afternoon the sunshine is too cruel especially if you had to gain a lot of altitude. My suggestion: start at 6:30 am until 10-11 am and then Siesta!. Later from 4 to 6:30 pm you should reach your destination much easier.