There are countless minivan services for hire from nearly all guest houses and travel agencies. You can get transfers between the larger Don Khong island to/from the smaller islands in the south of Don Det & Don Khone for about 40,000 kip. Travel time is about 30 minutes. (not including river ferry).
However if you want a more authentic experience, take the ~2 hour slow boat journey between the two islands. You can see fishermen, birds, kids playing, as well as see the architecture of the temples and homes from the water!
Most hotels will offer the boat tranfser (round trip) for 100,000 kip/p to 150,000 kip/p. However if you ask around and negotiate, you should pay no more than 75,000 kip for round trip.
You can travel to Pakse by public bus. However, the cost will be nearly identical to a mini-van ride if you find a good price. But the real savings will be time! The public bus will take ~40 minutes longer than the mini-van, and there is no idea how long you will have to wait on the side of the road waiting for the bus since there is no regular schedule!
If you want to take public bus, take the ferry from Don Khong (10,000 kip) and walk the 0.8km to the main road and wait for the bus (30,000 kip) and then take a tuk-tuk from the "8km Bus terminal" (15,000 kip).
However, if you want a good mini-van, price will include the ferry and drop-off to any location in Pakse as you wish! Call "Phoumy" at the phone listed below, or ask any guesthouse owner to contact him since he knows about everyone there. Departure is at 11:30 am, cost is 50,000 kip.
Again, there is not much public transport in southern Laos. There are one or two tuk-trucks heading to Veunkham (border crossing for Cambodia) but usually you'll have to hire a vehicle. The tuk-trucks that do run do so early in the morning. I wanted to see the waterfalls so I hired a motorcycle for $8 and took in the falls on the way to Veunkham. I could have had a truck or car, but that costs more and since there's no traffic, a moto isn't as big a health risk as it is in most places.
At Veunkham, you proceed through Laos customs (more on that in the Laos country chapter) and take a longboat across the Mekong to Cambodian customs. See the Cambodia and Stung Streng chapters for futher info.
There isn't much public transport around here. I got the hell out of Champassack with a quickness, on account of bad experiences with attempted scam artists there. I crossed the river but there were no tuk-tuk drivers to take me the 4-5 km to the highway. I decided to walk. A sweating mess, I gave up halfway and jumped in the back of a pickup.
At the highway, a few minutes later, a bunch of other travellers arrived. None of us had any idea when the tuk-truck would come by, so when a minivan stopped and offered a lift, we negotiated a price and hopped in. We ended up paying $3-4 each - a little steep for the region, but it was very fast and air conditioned. From there, the longboat across the island costs 5000 kip - negotiation is futile as the longboat skippers have a bit of a cartel going. All the accomodation is right there when you get off.
To Don Khong you can easily travel. At several places you find boats that will take you to the mainland. If you travel by car you can take a carferry, otherwise you can just hop on almost every boat.