I'll second that suggestion - the Intercontinental by JNB airport (NOT the one in Sandton - make sure of that) is the way to go for easy in-out of the airport without needing to go into the city.

If you call them, they'll send someone (a bellboy) to the airport to help you across the street with your bags - but in all honesty you can just take your trolley yourself. It's barely a 1-2 minute walk outside of the airport. Highly convenient and a very nice hotel itself..