the obvious answer of course is Amsterdam, but you might also think about Germany, especially Berlin and or Munich, as i don't see that you have yet made it to any Germanic cities.

your travel "CV" is also lacking in italian cities, which might be particularly nice in early autumn - fly to Venice, train to Florence, train to Rome, fly home. a perfect trip for 10-14 days.

Have a look at www.skyscanner.net and use its flexible searching options: search from Brussels to "everywhere" for the ENTIRE month of September. It will show you the cheapest flights to anywhere in Europe - - then let your imagination be your guide.

If something catches your fancy, then you can choose to search for flights from THAT city to "everywhere", and let the game continue.

If you put together an itinerary that way, be aware that it is very likely that your ticket to Brussels can be set up as an open jaw (e.g., from the US to Brussels; return from Rome to the U.S.) at a price not too different than flying round-trip to Brussels - - that way you won't have to double back. Just be aware that most intra-European airlines will only allow checked baggage weighing max 20kg.

It is possible that some budget carriers have not yet loaded their September flight schedules, so you might want to also try the search for the month of May, to see what other options might crop up if you wait a month or two to book.

Italy is conspicuously missing from your list and so is Greece, but you certainly don't have time to see both.

I would choose one or the other, and I think you will benefit from not only seeing either of these beautiful, fascinating countries, but also storing up plenty of sunshine and Vitamin D.

I would either pick Rome plus Napoli and the Amalfi Coast, or Athens plus something of the Peloponnese, or greek islands.