If you are fortunate enough to come to my neck of the woods in beautiful Johannesburg (the largest man made forest in the world) and have only one day to spend touring this is what I would recommend you do:
Start your day out at Maropeng, the Cradle of Humankind. It is a beautiful and world class exhibit.
Get your fix of wildlife at the Rhino and Lion park (You can have a meal at a restaurant that overlooks the giraffe enclosure.)
Meander through the Walter Sisulu Botanical Gardens. (Take the forest path as you walk to the Witpoortjie falls). Walk back along the main path and have a look at the geological garden it is quite a display.
Heading back into Johannesburg stop in at the Constitutional Court – it has an amazing collection of art. (There is a free evening tour on the last Thursday evening of each month).
Go to the Carlton Centre and have a look at the amazing views. In fact just take advantage of the hop on hop off city of Johannesburg tour.