The structure
Pick two estates, not five, and put a long lunch between them. The Winelands reward slow over thorough. Two serious tastings, one unhurried meal, and a drive through country that's the point. Trying to do four estates in a day is the most common mistake. You taste nothing by the third and you're driving dusk roads with wine in you.
A good one-day loop
Morning in Stellenbosch. A serious estate (Vergelegen or Tokara for the views, a smaller one for the wine) and a walk through the town. Long lunch at a farm restaurant: Babylonstoren, Jordan, or Delaire. Afternoon in Franschhoek. One lighter tasting and a stroll, or the wine tram if you must, but the tram is tourist-busy; a single estate with a view is better.
The long lunch is the point
The Winelands' best single experience isn't a tasting; it's a farm lunch at midday, a few glasses, and the afternoon to recover. Babylonstoren's garden and table, Jordan's lake, Delaire's view over the valley. Pick one, book it, and let the tasting be the thing you do around it. A lot of people fit in five estates and remember none of the wine. The ones who do two and a long lunch remember the whole day.
Don't
Drink and drive. The limit is low (0.05) and the back roads are winding. Spit or share, or book a driver. A guide for the day costs less than you'd spend on wine and is the difference between a good day and a risky one.
Make a day, not a sprint
The Winelands are an hour from Cape Town. You can do a day trip. But two nights, one in Stellenbosch or Franschhoek, turns the wine country from a box-tick into the part of the trip you talk about most. The valley at dusk, after the day-trippers leave, is the part people come back for.
The half-day, if that's all you have
Constantia. Twenty minutes from the city, three or four estates you can do in an afternoon, and the one to pick if you have a free half-day in Cape Town and no full day to give the Winelands. Not the same as a Stellenbosch day, but the best wine you can reach without losing the whole day to the drive.