Ghent and Bruges are must-visit day trips from either Brussels or Antwerp, easily reachable by train.
Ghent was at the heart of the cloth trade during medieval times. They manufactured luxury cloth from imported English wool and became famous throughout Europe until the 15th century. This brought a lot of wealth and autonomy, but heavy taxes caused resentment and uprisings. Coupled with changing boundaries, Ghent fell into decline.
Today, this place has become a tourist magnet thanks to its medieval architecture. There is enough to fill an entire day so it will be a bit too ambitious to combine a visit with Bruges together.