IC at the airport is nice and bellman will take your bags to airport check-in but otherwise quite boring for a longer stay. Sandton has some fun shopping and restaurants and is fairly safe. You don't want to rent a car unless you know what driving in South Africa is like (such as what areas to avoid and if you happen to drive there accidentally, don't stop at red lights). In any case no need for a car in Sandton, everything you can do in a day stay is within walking distance. Of course Sandton is a completely artificial environment too, and you barely know you are in South Africa (more like Italy from the architecture), so staying at the airport is not a bad option if you can entertain yourself in your room.