The first recommendation I was given for Cambodia was for Crusoe Island a year or two ago. It's a resort aiming to give the simple life. Hammocks, no electricity (other than a few lights run from solar power) and no running water. On the island there are about 4 other resorts and there are regular rumours that everything will soon be replaced by expensive resorts and an airport after Chinese investors bought the island. However, it doesn't look like that's going to happen any time soon.
Accommodation
Crusoe Island
You get picked up by a boat from Sihanoukville and taken directly to the resort. To call it a resort definitely feels wrong. It's a very small area of beach and forest, with options to sleep in a tent, hammock or a 'bungalow' (without walls, it's basically just a roof over your head and nothing else). There's no running water so showers consist of getting water from a well and then using buckets to pour slightly muddy water over yourself. It's actually not that bad! If you want a relaxing break and to sit in a hammock and read, this place is perfect! Obviously you need to be ok with the basic lifestyle. Thanks to my childhood spent camping, I really enjoyed it.
Food
The food is cooked by the Cambodian wife of the Australian owner and her family. It was all tasty! Most nights they have a special which most people order and it was always great. Apparently there are also times when there is a big communal bbq but it never happened during the 4 nights I was there.
Nightlife
There's a bar and a communal area. We sat around drinking and chatting every night but it was all very relaxed. I guess it depends on the type of people that are staying there. The maximum number of people I saw was 10 on the first night, but it wasn't exactly high season. If the people are nice, you'll have a fun time.
Sights & Activities
Fishing trips
The owner can take you fishing, although the guys who went out on a very early morning trip were unsuccessful. I went out for 10 minutes and that was also fruitless but it wasn't enough time.
Jungle trek and sunset cruise
For a few dollars you get guided through the jungle to the other side of the island, visit a fishing village and then have the option of jumping off a naval barge from 46ft. I jumped twice and loved it! You then get the boat back at sunset. Recommended!
Absinthe distillery
Unfortunately it was closed when we visited but apparently the owner is worth the walk along the beach along. We still got to see all the equipment, which was nice.