I would highly recommend the whitewater rafting! It is definitely an experience, as there are some great rapids, mostly grade five's I think. We flipped over at No 18, which they call 'the mother of all rapids'. Its a real rush! They have high safety standards and experienced crew, so although it is dangerous, its still worth it! The scary bit for me was when I went into the water when the raft overturned, as the water's very fast. I bashed myself against a rock underwater, but luckily, it was smooth (obviously, but you dont realise these things in the situation... )
and I was soon hauled out! 
And of course, there's no other way of experiencing the power, grandeur and beauty of the Zambezi from below the Falls.
