Houay Xai
This small town at the Mekong is where a lot of people arrive from Thailand to take the popular boat route to Luang Prabang. There is not an awful lot to see but check out the small Wat.
Luang Namtha
Luang Namtha is located near the Nam Ha protected nature reserve. The town consists of 1 street and is a base for one or more day treks, which are offered by various agencies, often with overnight stays in villages inhabited by minorities. But it also pays to go out on a bike or motorcycle yourself.
Nong Khiaw
A nice small riverside town with plenty of decent accommodation. There are walks to waterfalls and boat trips to small villages.
Plain of Jars
This area, which lies on the Xiangkhoang plateau, has been called the epicenter of the American bombing raids on Laos. Therefore, be sure to visit the museum that highlights that period. But the region is best known for the thousands of stone jars that can be found in groups in the landscape. The sometimes nearly man-sized pitchers, dating from 500 BC to 500 AD, are associated with burial practices.
Vang Vien
An outdoor paradise equally visited by young backpackers and Asians. Nice karst mountains, ponds and waterfalls. Take a pushbike.