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.

Bruges for the (somewhat cliché and touristy, but for good reason) medieval vibe. Ghent or Antwerp for hipster cities. Don’t underestimate Brussels too, it’s a bit tougher to love but there’s loads of stuff to do.

Every city has their own places that are interesting to visit.

I suggest looking for some places in the closest city to you, and then consider stuff in other cities.

It mostly depends on how much time do you guys want to spend visiting stuff.