Brussels is a good city if you stick to the downtown areas and avoid the area around Bruxelles-Midi, which is a bit rough. There is a drug war going on which is playing out in Curegem, which is in the commune Anderlecht, but as a tourist you are never likely to be going there. In spite of this there is a lot to see and do in Brussels. If you have time for nothing else (i.e. you want to see Ghent, which is entirely reasonable), I would still recommend seeing the Grand' Place. At night it is lit up and quite extraordinary, so you could sneak in for one of the evenings you have in the city, and perhaps drink a beer at Roy d'Espagne, which is on the GP. I think if you are going to commit to Ghent, you will want a full day there. It is hard to prioritise amongst the 4 cities (like children, I don't like to play favourites).
I hope you have a fabulous cruise. April is an awesome time to be in Belgium. There are some neighbourhoods in Brussels with cherry blossom trees and it is just beautiful, although you have to know where to go. The Royal Greenhouses are also open; you need a ticket, which is by ballot, and there is a wood near Hal(le) (Hallerbos) which has wild bluebells. Lots to see if you have the time and are so inclined.
Lavandula