I don't have experience with day hotels, but there are some listed
here.
There are other options to "chill out" if you don't need a room and bed. Will you have access to any lounges at JNB? You can purchase a pass to the Shongololo Lounge, for example (this would be after you've gone through security for your departing flight). If you're leaving the airport, you could take the Gautrain to Rosebank, a district with upscale shops and restaurants.