If there is a stop not to miss between Kruger national park and Johannesburg, that is definitely Blyde River Canyon. The viewpoint is about 70km from Kruger in the Mpumalanga province.
It is one of the largest canyons in the world! the views over the 16km long, 800m deep canyon are truly fantastic! It is actually the 2nd most visited natural attraction in South Africa, after Table Mountain.
I mean, look at that!! … some pictures of Blyde River Canyon: