Ghent or Antwerp?

For a day trip I'd usually pick Ghent, which is a more attractive city than Antwerp and slightly more walkable (although Antwerp isn't bad). The only attractions I'd suggest visiting Antwerp for specifically are the Rubens House, and the Plantin-Moretus Museum of books and printing. If one of you is a bibliophile I would definitely recommend going to Antwerp for the Plantin-Moretus, but only if that's a major interest.

Both are very easy to visit as a daytrip from Brussels and well worth your time. To compare and contrast... I would describe Antwerp and Ghent as the NYC and Boston of Flanders, respectively. Antwerp is a large port city, both gritty and elegant at the same time. In addition to the Rubenshuis already mentioned, the soaring late Gothic cathedral here is one of the most overlooked in Europe, in my opinion. The Grote Markt, to me, is even more impressive than Brussels' more famous example. If your interested in maritime history and culture at all, there's a nice little sailor's museum in a small castle on the bank of the Scheldt river. The elaborate train station is almost worth a trip to the city in itself. There's also a very long pedestrian-only shopping street. The historical center of Ghent is similar to Brugge, but unlike Brugge's center, the economy doesn't depend primarily on tourism. There's a large medieval castle (Het Gravensteen), an important altar piece in one of the cathedrals (The Adoration of the Mystic Lamb), one of the most beautiful streets in Europe (Het Graslei), a large public park, a pretty good municipal art museum and a large student population to keep things shaking. You can't go wrong with either choice. For a preview of each city, you can see clips of two Flemish TV shows that take place in each on Youtube, LouisLouise for Ghent (listed as "Lo & Th", probably to avoid copyright issues ), and Zone Stad for Antwerp... although being a police drama, it doesn't usually show the city at it's prettiest.