The Castelli towns sit on a ridge facing Rome, which means that at the end of the day you can stand on a terrace with a glass of something local and watch the light go off the city below you. It costs about as much as two coffees in the centre.
The evening also solves the summer problem. From June to September the sensible day out starts late: you drive out at five, the heat breaks around seven, the town comes out onto the street at eight and dinner is at nine because that is when dinner is.
There are night versions of several of the things on this site — visits after dark, a quad ride up to a square for pizza, thermal pools in the evening. The one practical point is the drive home, which is the strongest argument on this entire site for having somebody else do it.
The reason the evening works so well here is geographical rather than romantic. The Castelli towns stand on a crater rim three to seven hundred metres up, facing north-west across the plain at Rome, which means the whole city is between you and the sunset. From Rocca di Papa, Monte Porzio or a Frascati terrace you get Rome in silhouette and the sea behind it, and it happens every clear evening whether anybody has booked it or not.
What that buys you in summer is the part of the day nobody enjoys, skipped entirely. What it costs you is that the evening is when a good deal closes rather than opens — so the second grid below is the one to book from, because those are the operators who have told us they run late.
There is a real list of things that only run late, and they are in the second grid: a thermal pool open into the night, a floating aperitivo on Lake Nemi, a quad ride up to a square with pizza afterwards, a torchlit garden at Tivoli, a sunset boat off Ponza. Those are the operator's own timings rather than our assumption about when a thing would be nice.
And then the drive home, which is the honest weak point of an evening out. Unlit country roads, a glass of something, and the way back into the city all arrive at once. This is the single strongest case on this site for having somebody else drive, and split between four people it costs less than most people assume.