I went to Lille, France, from Antwerp, and it took two changes and about three hours. I got what I wanted, which was their famous art museum, but not much else in one day. It's a pretty city, nicknamed "Little Paris."
A city not mentioned is Lier, which is particularly attractive for day-trip by public transportation. It has small canals, some quirky little museums, and a local pastry specialty. Even the town-hall art museum has some interesting pieces, like marquetry cabinets and paintings by Rik Wouters' best friend in art school, Since you likely won't be going to some of the more westerly medieval town centers, this might be nice. It's true that Gent makes a fuller day, with more important art. Science and engineering students might particularly like the hand-built, building sized astronomical clock in Lier. AFAIR, it's dozens of specialized faces included a few religious touches.
If you want to do two cities in one day, Lier and Turnhout are doable. Turnhout is a minor town, but is surprisingly attractive, and has good restaurants, if you can delay your return until after eating. It has a lovely Beguinhof, with an old-folks center where you can get coffee and cake, as well as it's own museum.
In the unlikely event you get tired of Gent in less than a day, Mechelen has a magnificent cathedral and medieval square (Saturday market), some good museums, and fine churches, and is on the way. (Some walking from station to square.)