Twenty-four hours is enough for one neighbourhood done properly, two major sights, two decent meals, and one evening worth having. It is not enough for the Colosseum and the Louvre on the same day. Knowing the difference is the entire skill.

What Actually Fits

Arrive the evening before or very early morning. If you arrive the evening before, you get a bonus evening in the city, which is the most enjoyable way to spend it: no agenda, a drink, a walk, a meal. If you arrive early morning, you have a full day. Either way, the structure is the same. One neighbourhood as your base of operations. Two pre-booked sights. Lunch somewhere good but not complicated. A proper evening meal. That is 24 hours used well.

The sights need to be either in the same neighbourhood or genuinely close to each other. Do not plan to visit something on opposite sides of a city on a 24-hour trip. Transit time eats your margin and you arrive at the second sight already tired and rushed. Two things within walking distance of each other, or a short metro ride apart, is the correct brief.

What Does Not Fit

The Colosseum and the Louvre are in different countries, but even within a single city the same principle applies. The Prado and the Reina Sofia in Madrid, the Rijksmuseum and the Anne Frank House in Amsterdam, the Uffizi and the Accademia in Florence: these are all plausible-sounding combinations that will make your 24-hour trip feel like a forced march. Pick one. See it properly. Have lunch. Then do something less structured.

The other thing that does not fit is a late start. A 24-hour trip with a 10am first movement is a 14-hour trip. Be out by 7am or 8am at the latest.

The Meals

Do not spend 24 hours eating at whatever is nearby. Book one meal in advance, either lunch or dinner, somewhere that reflects what the city does well. The other meal can be more casual. A 24-hour trip with one genuinely good meal is far more memorable than two adequate ones.

The Evening

The evening is the part of a 24-hour trip that people most often get right instinctively. A drink in a bar, a walk along a river, sitting in a square. This is not wasted time. It is the reason you came. Protect it from over-scheduling.