Sorry but it's so vast, you're better off getting a really good map, marking all the elevated points, ie drakensberg mountains, hottentots holland, lesotho. From there you'll find the rivers, and flat lands, ie colesburg, the Karoo, Even the Garden route from Cape Town to Port Elisabeth, is alive with adventure, Tstistikama, A good place to look for secret spots would be the Western Province / Eastern Province border line, it runs from the coast up into the mountains with some very rough terrain, mountain roads, goat tracks.
As I say, you're better off getting a really good map and get stuck into a deep adventure mission.
And thats only the western province, there is so much more further north....
Where does one start to explain...