It's impossible to even consider organising a trip to South Africa without including Kruger National Park on the itinerary.
There are large numbers of animals at Kruger and you'll have a golden opportunity to see the Big Five there. The park also offers different kinds of landscapes across its extent, from its south to its north, and through its central part. Due to Its immense size it feels calm and peaceful, despite the large numbers of visitors. Though it's normally very expensive to stay at accommodation inside the park, this cost can be reduced by booking a place at one of the camp sites well in advance.
In addition to the classic safari in a specially equipped vehicle, I also recommend going on one of the walks accompanied by a ranger. It's a once in a lifetime experience.