This is the whole problem with a self-drive Winelands day, and nobody mentions it until you're standing in the first tasting room doing the maths.
Renting a car is easy enough. The N2 out of Cape Town is a good road, the estates all have parking, and South Africa drives on the left if that's what you're used to. But if you're planning to taste properly at three estates, one person in your group spends the day watching everyone else drink. For a couple, that's half the party.
There's no public transport worth the name between the farms. Uber will get you from Cape Town to Stellenbosch, but it falls apart once you're trying to move between estates that are ten minutes apart down farm roads with no signal.
So the day works out to one of two things: someone doesn't drink, or you book someone to drive. I'd suggest the second, and not only because it's what I do for a living — the day is genuinely better when nobody is counting their tastings.