Stung Treng (also Stoeng Treng) is a small town in northeastern Cambodia, and can easily be described with the word "outpost" more than anything else. Many tourists heading to/from Laos pass through here, though few stay long as Ratanakiri and Kratie offer far more excitement, activities, and remedies.
Yes, Stung Treng does have what could be considered an airport, and Royal Phnom Penh Airways flies here for $50 one way, $95 round trip, for a journey taking about an hour. Though this is a fast way to get to Cambodia's remote northeastern provinces, it's a much better idea to head up here either overland or by boat, because there's so much to see in between here and Phnom Penh. Either option is also far cheaper than flying out here as well, so you should generally avoid the airport here unless you are pressed for time and want to get out to one of Cambodia's most remote regions.
Fast boats are probably the best way to get to Stung Treng based on the absolutely ridiculous state of the highways in this part of the country, but to and from Kratie they only run from July through to December. The cost of the trip is $8.
Trucking from Kratie is an option, and will set you back 20-25,000 riel for riding in the back of the truck, and 25-30,000 riel if you wish to ride in the cabin. As discussed on the Kratie page, trucking is somewhat less safe than other modes of transport, and can take much longer. However, trucking in Cambodia puts you in much greater direct contact with the locals (as it's locals you'll be sharing a truck with most of the time), and isn't that one of the purposes of travelling?
Since the town is so small, there's no point in using anything other than your own two feet to get you anywhere within the town itself. If you decide to see any of the area's limited sights, simply hire one of the few motodops around for no more than a couple of thousand riel.
Most tourists who end up here do so only for a couple of hours before being rushed on south to Kratie or north to the Laos border. But, like anywhere in Cambodia, you could easily spend a day here relaxing by the river (don't plan on a week here, of course). There are a couple of sights to see here, but not enough to seriously keep yourself occupied.
Since the town is so small and out of the way, there isn't a whole lot to do in Stung Treng. As mentioned above, one day is enough here.
There are no banks in Stung Treng, and no place to get Travellers Cheque's cashed or changed, so make sure you've enough cash to get yourself to Kampong Cham or somewhere else where you can get more. Your guesthouse will be able to change money from USD to riel for you if you should so need.
From here you can head up on to the Laos border, but there are no regularly scheduled modes of transport there, so you're going to have to hire one yourself. The best option, again given the state of the roads in Cambodia, is to hire a fast boat up to the border crossing of Vueng Kham, which should cost about $20. These can fit usually five or so people, so if you're in a group it wouldn't burn nearly as great a hole in your pocket.