Cambodia's "Wild East" contains some of the most remote yet remarkable areas in the country and the vast majority of the region is well and truly off the beaten tourist trail, yet the majority of backpackers and travellers who get to this part of Cambodia see solely the Mekong riverside towns of Kratie and Stung Treng -- the former for the Irrawaddy Dolphins, the later for its proximity to Laos.
But for the more intrepid, both Mondulkiri and Ratanakiri are excellent destinations well worth exploring. The former is well regarded for its elephant trekking, waterfalls and jungle trekking potential, while the latter is the heart of Cambodia's Wild East, with a particularly remote national park, a crater lake, trekking, boat trips and even a new border crossing leading to neighbouring Vietnam.
The transportation infrastructure in the this part of Cambodia is undeveloped. The road as far as Stung Treng is reasonable, but the dirt roads to Sen Monorom (Mondulkiri) and Ban Lung (Ratanakiri) are pretty poor and the road between Ban Lung and Sen Monorom really isn't a road at all -- more like a goat track. As the roads have slowly improved, the passenger ferries that once plied the Mekong have finished running on a regular basis. All this means that in dry season the going can be slow and dusty and in the wet season, slow and muddy.
Despite the primitive transportation, the region is absolutely worth visiting. Aside from the dolphins and riverside scenery, you can visit volcanic crater lakes and exotic minority cemeteries in Ratanakiri and waterfalls and elephant riding in Mondulkiri -- all the time soaking up an interesting and very hospitable part of the country.
If you have the time (time is important in this part of Cambodia, as the going is very slow) set aside a week or so in this part of the country -- you won't be disappointed.