If you like landscapes, the south has a bit of shape (not really mountains) and the north is mostly flat, in parts spectacularly so (around Veurne, for example). In april there should be blooms in the fruit country around St-Truiden. You can get bicycles at the railway station and enjoy the countryside for a day.
If you don't mind milling about among hordes of tourists, Bruges is nice, but Brussels, Antwerp, Ghent, Liège, Charleroi, are the bigger cities, each fascinating on their own account. Antwerp is a port in flatland, Liège sits around an island in a river with cliffs on one side, Charleroi is surrounded by terrils, man-made mountains of coal-mine remainder, and other archaeology of the industrial revolution, and Ghent has an honest to god castle. Thousands of things to see. And everything connected by train.
You won't have enough with three weeks, not even three months. You might get bored after thirty years, but then there is always the beer. You'll need to try a few. We have three or four hundred. Take care, people will suggest the stuff that gets you dancing on the table and singing lewd songs very, very fast.
I don't know yet what my work schedule is for april, but if you want, I could maybe show you around Brussels a little. Send a message.
