The Otter Trail at Tsitsikamma is probably SA's most well known backcountry trek. It's a five day, 45 km trek along the south coast. I haven't done it completely, but I have day hiked parts of it. It's quite popular so you have to book well ahead. It's beautiful, much like our West Coast Trail. There is wonderful hiking in the Great Karoo and the Drakensbergs. I've hiked in the central Berg and northern Berg, but not the southern berg. There is some great hiking in Lesotho, near Sani Pass. That area has the highest death rate from lightning strikes in the world, so some care should be taken there. There are a couple of small towns, Sabie and Graskop that offer tons of hiking and mountain biking, adventures of all sorts. Of course in Cape Town you can hike up Table Mountain, well worth it. Upington in the north has some wonderful hiking, but they have some very dangerous snakes, like the puf Adder.
If you need any info on cheaper backpackers, I've stayed in many, or anything else, you can email me.
I wish both you and your husband and Dav and his wife a wonderful trip. It's a truly amazing country. It's tearing me apart because I very much want to go back again this year, but my wife and I have decided on the Indian Himalaya and will be skipping SA, unless of course, those lottery numbers finally hit. LOL