The province of Stung Treng marks Cambodia's northern border with Laos and the point where the Mekong river enters Cambodia after swirling its way around the 4,000 Islands of southern Laos. Originally a part of Laos, Stung Treng switched to Cambodian ownership during the French period and to this day you'll hear a lot of Lao spoken in the same-named provincial capital. The quite large province stretches out from both sides of the banks of the Mekong, though just about all travellers to this part of Cambodia stay on the east bank, at the capital, which sits on the southern bank of the San River.
The province holds considerable promise for those with an adventurous spirit (and a fair amount of free time) as a number of riverine trips can be undertaken, and you can explore the western bank of the Mekong that very few bother with -- motorcyclists in particular rave about the off-road opportunities in western Stung Treng en route to Preah Vihear.
For most travellers however, Stung Treng is but a glimpse from their bus window as they're shuttled to and from the Lao border and the larger town of Kratie to the south. Around 23km south of Stung Treng town, the road east to Banlung in Rattanakiri begins, so overlanders coming from Laos but bound for Banlung will need to pass through Stung Treng. As with many of the low-key destinations in Cambodia, the capital can be a little underwhelming, but give the place some time and you'll find enough to keep yourself busy for at least a day.
Stung Treng doesn't sit on the bank of the Mekong, but rather on the southern bank of the San River which flows into the far larger Mekong about a kilometre west of town. The town itself is a rather rundown affair, with its major asset being spectacular sunsets and friendly people. The one attraction of sorts is a weaving co-operative at the eastern extremity of town, which is worth visiting both for its lovely fabrics and to support a worthwhile cause.
Aside from the co-operative, the main pastimes are boat trips and watching the sunset -- both of which are worth doing. One-hour boat trips leave from the dock in the centre of town and cost around $15 per person, or $30 for a whole boat. There are some homestay options as well as joint boating/trekking trips to nearby temple ruins, one set of which dates back to the pre-Angkorian Chenla period. Trips vary according to who's around to guide you, so ask at Riverside and other guesthouses.
More interesting kayak and bicyle trips north along the Mekong to two stretches of semi-submerged forests are worth looking into, though prices are quite high for solo travellers. Given many travellers are now only transitting through town rustling up a group can be a challenge.
A tourism office is located east along the river road from town, but it's often closed and there's no real tourism industry to speak of, other than a few guys who've figured out what foreigners like to do. Your best bet for information and improvised boat trips to see dolphins or take a motorbike trip north into Preah Vihear is to stop by Riverside Guesthouse and talk to the guys who hang out there.
Another option is to visit the Mekong Discovery Trail website, a joint government/NGO project which aimed to boost tourism to the northeast Mekong region. It produced a glossy brochure and a boatload of ideas for trips, some of which are ambitious -- we'd say unrealistic -- along the river and throughout the province. You'll need assistance to complete some of the recommendations but if you use sleepy Stung Treng as your base, you're virtually guaranteed to see a slice of Cambodia untainted by mass tourism.
Of all the towns along the Mekong, Stung Treng is the most likely to wither on the vine from a tourism point of view, but, if you want to see a slice of unadulterated Cambodia, you could do far worse.