See if you can book "Le Pique Nique" in Boschendal (www.boschendal.com) - very relaxing way to spend an afternoon with good food and wine, under an hours drive from Cape Town. Could combine it with a wine tasting there too.
We went to who only make sparkling wines. I'd never had red sparkling before so it was very interesting to try. For a ridiculously low price we got to try 6 different wines. There is a restaurant there which is open for breakfast and lunch.
Beyerskloof vineyard is also close to Stellenbosch - www.beyerskloof.co.za
They make a really nice pinotage which I'd highly recommend.
I believe they now have a restaurant on site too.
I agree with fandango1. Beyerskloof pinotage is a wonderful wine which sells for £6.99 in our local Tesco.
Spier, near to Stellenbosch also produce some great wines, and also have lots of birds (including vultures) for you to mingle with. In addition,you can also stroke a cheetah - provided that you do it carefully!
Just happened to call into a shop in the main st in Stellenbosch when I was there to enquire about a visit to a local township. The guy in the shop (an Englishman whose name I can't remember) was able to organise that for me. As it turned out he also gave vineyard tours himself which I did teh following day - great guy & super day visiting 6 or 7 vineyards. the shop is about half way up Victoria St and sells maps and the usual tourist stuff but if you can find him I would definitely recommend him.
I lived down there for a bit so know exactly where you need to go. If you need any travel advice please feel free to ask! Haven't been to Stellenbosch in years and thats why i asked about wine tasting to see if there were some new suggestions!