Zimbabwe
The World Heritage Site of the Victoria Falls National Park is an extension one could easily add on your South African/Namibia/Botswana safari, or if you had more time, a visit to Hwange National Park further south.
Lesotho
This small landlocked kingdom encircled by South Africa is crisscrossed by a network of rivers and mountains. A lovely attraction is the Drakensberg Mountains with its Sani pass, or near the capital the ruins from the 19th century reign of King Moshoeshoe.