I agree that exploring where you will be docked/staying is probably fine. I like Antwerp better than Brussels, but for the first time visitor, Brussels has more "known" sights and a wider array of museums.
For Brussels, if you have specific interests, then take a look at the museums. In my view, I do not class Brussels museums as any that are "must sees" like you would find in Paris, Rome, Amsterdam, and Madrid, but they do have some neat niche museums for Comics, Musical instruments, the works of Rene Magritte, Autoworld, a good Military museum, some Dutch/Flemish Masters, and more, you just need to have an interest in the subject of the museum.
As for Luxembourg, I assume you will be in Luxembourg City. If so, just wandering the town, maybe stopping for a snack will fill your time. It is a neat old town, some good views. We didn't go, but a tour of the Casemates du Bock, underground areas used for protection and storage for centuries, is popular.