We've just come back from a three week trip, get researching what you'd like to do and just book up using websites like Hotels.com and Expedia etc, or once you've found suitable places try approaching them directly as you might be able to negotiate a good deal. It's also worth signing up to the Cape Town Groupon for special deals.
It can be cool and windy in July / August and outdoor pools aren't generally heated. We had mostly fabulous warm sunny weather, but it will be cold up in the hills and we had a couple of days of heavy rain.
We stayed in hotels, guest houses and self-catering apartments. Flights will be expensive unless you take a longer route via Istanbul or the ME.
We booked car hire from the UK direct with Avis - a Toyota Corolla was plenty big enough for 3 adults and a child, and works out at about £15 per day.
Cape Town: stayed at the V&A Waterfront, safe for walking around in the evening and lots of restaurants to choose from. Heaps of things to do - cable car up Table Mountain, trip to the Cape, planetarium, aquarium, Boulders Beach and Betty's Bay for penguins etc.
Hermanus: whales were already in the bay, and DH & DS went shark cage diving at nearby Gansbaii with Great White Shark Tours (DS was most excited that a great white bit the cage right in front of him). We stayed at the Whale Coast Hotel - SC apartments, really good.
There are loads of private game reserves, from cheap and cheerful to luxury and expensive - it depends what you want to see. We stayed at Nyaru near Mossel Bay and had a fab day doing game drives and quad biking (we were the only people staying so got exclusive tours). Nearby Gondwana is apparently very good (when they're not losing lions!) and Buffelsdrift at Oudtshoorn was also recommended to us. If you can make it as far as Addo then go for it - you'll see elephants up close, as well as lots of antelope and zebra etc. You can stay in the park (book via Sanparks), the restaurant at the Main Camp is very good. Shamwari is close to Addo - expensive though - and our guest house also recommended nearby Schotia (you can book day drives if you don't want to stay overnight). These are all malaria free areas.
Closer to Cape Town there is Fairy Glen near Worcester, and Inverdoorn and Aquila. Sanbona has children's activities.
The Daniell Cheetah Project is also worth dropping into - not often you get the chance to tickle a cheetah and make her purr!
Plettenberg Bay is worth staying at, as there are loads of animal things to do - Monkeyland was good fun, there are also elephant parks and big cat sanctuaries there.
Don't miss Storm's River Mouth and the rope bridges, and your DH can do the bungy jump off Bloukrans Bridge.
You can probably tell that we had a fab time!