

Most general “must-see” places have already been mentioned, you can’t go wrong with them. I would perhaps throw in Leuven as a good half/one day trip, especially if you’re staying in the center of the country.
I’d suggest to also focus on distinctly Belgian food/activities, which your friend might not have in Germany: visiting a frituur, sampling the local beer/pralines, going for a bike ride, perhaps a cantus if you’re up for that. These are independent of location but are all quintessentially Belgian.
The first game in the Kingdom series, “Kingdom: Classic”, is free on Steam (Linux, Mac, Windows), so if you’re undecided about the bundle you can try that one out first. I just finished it and can certainly recommend it; it has a very engaging gameplay, beautiful pixel art and simple, intuitive game mechanics.