U can visit beautiful places like Bruges and Ghent in Belgium... they are less than an hour away from Brussels by train. Paris, France is less than 2 hrs train journey from Brussels.
Wow tomorrowland .. awesome. good luck with ur trip. try to book paris as ur primary entry point and then go to amsterdam then brusells and may be go fro Switzerland or Italy
Brussels, Grand Palace, Bruges, Antwerp, Netherlands Amsterdam, Den Haag, Rotterdam, Paris (One Night Journey), Switzerland (Lucern, Interlaken, Zurich)
Austria Zell Am See, Vienna
Hungary Budapest
Spain Barcelona Madrid
Rome Italy Venice Milan
Germany Berlin, Dusseldorf (A day trip from Amsterdam)
Denmark Copenhagen Early Morning flights will be cheaper and save time on travel if you plan and book early.