I suggest Kong Lor village.
Beautiful amazing landscape, rice paddies, a flat valley, with undescribably formed rocky walls; dead-end-road to kong lor cave, so hardly any traffic. Old village with lots of homestays, my favorite guesthouse with the funny name "Enjoy Boys" right at the river with a terrace overlooking the river, also some other, a bit more modern, newly built guesthouses. One "restaurant" with reasonable internet. Lots of possibilities to explore the countryside, or the cave and beyond.
I have been to Nong Khiaw and also to nearby Muang Ngoi (a beautiful place as well), I have been to the 4000 islands and lots of places in between, but if I was looking for a quiet, beautiful place - more laotian than touristy, but with a few tourists - I'd go to Kong Lor.
A bus ride a little less than an hour brings you to the next village with a market (at the crossroads). A maybe 4-5 hours bus ride brings you to either Paksan or Thakek, both at the Mekong, Thakek being my favorite. There is a friendship bridge to Thailand, the Thai border town got an airport - so if need be there is a quick way to Thailand and Bangkok.
Whereever you go - I am sure, you'll have a great time in Laos :-)
Monika