If you head towards Ban Lung (Ratanakiri provence) in north-eastern Cambodia you'll find yourself well away from the main tourist haunts. There are some nice villages in the surroundings, waterfalls, volcanic lakes, national parks etc. It can be slow going to get to some of these remote parts and if it's the rainy season they can become impossible so you'd need at least a week to get there, see it and get back.
Your better off making your own way there and then talking to the locals about finding someone that will guide you around the area. You might have to forego some of your comforts as it's not that developed so maybe it's not everyones cup of tea. To back up what bogwarrior said about the landmines... if you don't walk off the well worn paths you should be fine; the locals live with the threat of landmines and uxos daily and just use common sense to avoid them.