Ghent's quiet advantage is geography. It sits on the main line between Brussels and Bruges, in a country you can cross in a morning, which means half of Flanders is a day trip and you never need a car to reach it. Nothing on this list is much more than an hour from the station, and most are closer.
That changes what a day trip is for. In most cities it costs you a day of the place you came to see. From Ghent it is closer to a menu: you can be in Antwerp by mid-morning, back before dinner, and still have Ghent's illuminated centre almost to yourself after dark — the part most visitors miss entirely, because they are asleep in Bruges.
The list runs roughly outward. The first five are the ones people have heard of. The last two are the ones they come home talking about.
Before you go
The station is not the centre. Almost everything leaves from Gent-Sint-Pieters, which sits well south of the old town — budget fifteen to twenty minutes on tram 1 from the Korenmarkt, and more on a weekday morning when the trams fill with students. A handful of services, including some running north towards Antwerp, use Gent-Dampoort on the eastern side instead. Check which station your train actually calls at before you pick a tram; arriving at the right platform in the wrong district is the classic Ghent mistake.
Fares are distance-based and there is nothing to game. A single to the places here typically falls somewhere between about EUR 6 and about EUR 16, and the price is the same whether you buy three weeks out or two minutes before departure. There is no advance-purchase pricing to chase. What does move the number is when you travel: the national operator has long applied an automatic reduction to weekend and public-holiday journeys, and runs a separate discounted weekend return, so the same trip can cost appreciably less on a Saturday than on a Tuesday. Reductions and railcards change from year to year — check the operator's site for what is current when you go. Buy in the app; the ticket-machine queue at Sint-Pieters on a summer morning is its own small tragedy.
Trains are frequent, so do not over-plan. Most of these routes run once or twice an hour into the evening. You do not need to fix a return train in advance, and you should not: the freedom to stay another hour is the whole point of a country this small.
Most of these are half-days. Belgian towns are compact and their sights cluster. Bruges, Leuven and Mechelen can each be done properly in four or five hours, which means two of them can share a day if you are willing to spend the transit time between them. Antwerp, Brussels and Ypres cannot. Do not try.