Everything here is inside an hour's drive of Rome and takes four hours or less once you are there. That means you can leave after breakfast and be back for a late lunch, or leave after lunch and be back for dinner — with the day out of the city being the thing you remember rather than the driving.
The Castelli Romani do most of the work on this page, because they are half an hour away and full of things that take an afternoon. Tivoli, Ostia and the near Tuscia do the rest.
The arithmetic that matters is not the distance, it is the round trip plus the thing itself. An hour out and an hour back is two hours of the day gone before anything has happened. So nothing on this page is more than about an hour away and nothing at the far end runs longer than an afternoon, which leaves a real half day rather than a drive with a stop in the middle of it.
Choose by which half of your day is free. A morning suits anything involving a town, a park or an archaeological site — that is when they are open and when the light is worth having. An afternoon suits a cellar, a tasting, a workshop, a kitchen or thermal water, all of which are better late and none of which mind the dark on the drive home.
The other half of the trick is not going back to the hotel in between. A car for the day, or a driver, is what makes a half day out and an evening in Rome fit together, because the thing that turns half a day into a whole one is not the distance — it is the waiting.