Africa is in the first place known as a safari destination and people visit to see the Big Five. But there is more than the African bush. Endless uninhabited beaches, spectacular mountains, beautiful coastal landscapes, exceptional sights like the Victoria Falls and lively cities are the overwhelming choice of African safari extensions.
There are two ways in planning your Africa holiday with the safari extension that will suit you best. It either starts with the activities you would love to do or with the geographic location of your safari. If you want to climb Mt. Kilimanjaro, it makes sense to plan you safari for Tanzania or Kenya. Or if you want to do your safari in South Africa, you might like to see the Victoria Falls.
If beaches are your preferred safari extension, you will be able to do that in East and Southern Africa, but you should mind the climate. When travelling in the southern hemisphere winter, you should not plan a beach extension for South Africa. It will be too cold, but there would be other attractions like the whales.
Essentially it boils down to what your interests are besides the safari. Take it from there and avoid too far distances between your safari destination and the chosen extension, as you will loose probably days with travel. Rather come back another time to see other parts of the continent.