Most people blow through the small village of Champasak on their way to see Vat Phou. There isn't much to the place, which is spread out along the Mekong River, but it does have its charms. There are a few nice hotels, guesthouses and restaurants around the town, which is sprinkled with some interesting historic architecture. If you're looking for a place that offers a more laid-back relaxing atmosphere, you might be wise to spend more time in Champasak and less time in Pakse.
Champasak is a natural place to stop between Pakse and the 4,000 Islands area. How much time you spend here largely depends on your interest in the life and architecture of the area, versus the more urban setting of Pakse or the natural charms of the 4,000 Islands. You can 'see' the place in a single day, but if you're on a relaxed schedule you may feel like spending more time here.