The popularity of rafting of any sort is gradually increasing since 1970s. It is challenging but you can always define the level of difficulty. According to the location and the season it may vary from calm, relaxing trip to really extreme adventure.
Some of the best locations for experienced rafters are Colorado River, USA, Magpie River, Canada, White Nile, Uganda, Ganges River, India, North Johnstone River, Australia, etc. But the good news is that there are spots for rafting almost anywhere. That means that you can try this type of activity any weekend. And some local rivers may be as unforgettable as more exotic destinations.