We stayed in Brussels and took day trips by train to Ghent one day and Bruges the next. They were easy trips and we chose to walk from the train station to the attractions in the city centers. A car would only be a hindrance.
In Ghent, the highlight for us was Jan Van Eyck's Ghent Altarpiece, properly called the Adoration of the Mystic Lamb, widely considered to be one of the most important pieces of art in the history of Western civilization. Before our trip we did quite a bit of reading and watching of lectures on YouTube videos to learn about the Altarpiece's creation, symbolism and meaning, and significance in the development of artistic techniques (it’s been called the very first true oil painting, for example). You can see the amazing detail of Van Eyck’s artistry by zooming in on the super-high resolution photographs on this website: Closer to Van Eyck
The Altarpiece is in St. Bavo’s Cathedral. We also went to the Museum of Fine Arts where some of the panels are still being restored. It was fascinating to watch the restorers at work, looking through microscopes as they examine and remove (with eye surgery scalpels, because regular surgical scalpels are fine enough) the layers of overpainting done through the centuries.
In Bruges, we especially loved Michelangelo’s sculpture, Madonna and Child, located in the Church of Our Lady. It’s the only one of Michelangelo’s sculptures that left Italy during his lifetime. Here’s one short video about it: SmartHistory: Michelangelo's Madonna and Child
Both the Ghent Altarpiece and Michelangelo’s Madonna and Child were stolen by the Nazis in World War II and are featured in the book and movie, The Monuments Men (which we watched again before we went). The Altarpiece has been stolen more than half a dozen times and has been called the most stolen artwork ever.
We aren't art/art history people, so the advance reading and watching made what could have been just another quick look at two nice pieces of art into something very special we will always cherish.
Hope you have a great trip!