South Africa is amazing, there is so much to do and such a range of different things to do and see.
We went in October (many years ago) and had lovely weather, perhaps we were lucky, but it was warm enough to enjoy being out doing everything and not too hot to make it uncomfortable. It wasn't quite beach weather if you know what I mean, but was perfect for doing everything.
I would say Dec & Jan, as well as being too hot, would also be too crowded, we found it easy to book things and get into things as it wasn't yet high season.
We based our trip in capetown, so that's where my tuppence starts...
On what to do, I'd have to say first Robben Island, it's something I'll always remember, I felt I had to see it when we were there.
This might sound an odd thing to say or a strange thing to do but it was one of the most memorable parts of our trip - a tour of one of the townships. Now, when it was first suggested to me I thought that's awful, invading people's privacy but actually it's not like that at all, we went on a tour with someone who is from the township, so it's very safe and you get to see the community projects that are going on and to support them where they have say, craft shops or something to provide jobs and training in the community.
The District Six museum was really good, it's in an area where different races had lived together without any problem until the apartheid government bulldozed the whole area, it gives a great account if what life was like before apartheid and then the insanity that followed.
We did a helicopter trip around Cape Town which was amazing, fantastic views, my one and only time in a chopper, a great experience, and we have amazing photos from it.
The food was excellent, eating was a big part of our trip! The steaks and fish are fantastic, in fact everything was lovely, we ate like kings.
We went to Stellenbosch wine region, spent a few days there and really enjoyed it, beautiful wine estates, a lovely way to spend a day going around a few on a wine tour!
Safari is a must, if we had our time again I'd push the boat out on this for two nights in a good one. They are expensive but well worth it. We had a brie (don't think that's the right spelling but a SA version of a barbecue) in a Bure (traditional house - not unlike a crannóg) and the food and atmosphere were fantastic.
Up the Garden Route Mossel Bay was lovely, and Cape Agulhas - oh and Hermanus! The whales were coming in to mate (or calve, I can't remember!) and in Bientang's Cave restaurant in Hermanus they were so close in October, I had binoculars on the table I didn't even need to use we could see them so clearly.
Haven't travelled with a group myself but two family friend who are aged around 70 have done a good few trips abroad with the Irish times travel - I think it's called The Travel Department? Anyway they have always enjoyed it and are repeat customers, they've gone to Argentina & China among other places with them.
Best of luck, it's a great place!