This was our most recent visit to South Africa, and we visited the Cape Town aquarium. It is quite big, and there's a lot to see, including people in deep see divers' outfits feeding the fish.
In the afternoon, we went to Robben Island to see where Nelson Mandela was kept prisoner during much of apartheid; it was a little sobering, but included an enjoyable bus tour. It seems that some people even live on the island now.