I've had another look back through old posts with advice for this. The general rule of thumb for daily spends is £30 per day, although Skippy's post suggests lunch, tips and a couple drinks should come to £10 a day but there's always shopping to consider! With that and the trips I'd like to do on the main tour comes to around £600 or approx 8000 Rand. I'm going to take the max 5000 Rand in cash and then put the rest on a Cash Passport. Travelex do one which I've used very successfully previously - you use it like a debit card - either paying with it or withdrawing cash from an ATM.
For the extension I am taking US dollars in cash (including $80 for the double entry visa). There is no restriction on the amount of foreign currency you bring into the country.
I will also take a credit card and a debit card as a back up. I think travellers cheques are fairly obsolete now and can be difficult to change.
I think you also asked about towels in a previous post. I recommend a trekking towel - this can be bought from either Millets, Blacks, Cotswolds etc and are fast drying compact towels. I have an extra large one which I always take with me but takes up very little space in my case. It can act as beach towel, an extra to dry my hair, a comfy cushion, pillow.....etc