Suggestions:

A) Spend 2 weeks in Italy…
B) Pick someplace close and easy to get to from Belgium or Netherlands, like Paris (add a stop in Strasbourg or Lille or take a side trip to Normandy) or London (again with a side trip or several day trips). In a week,you could visit London and Paris if you want to concentrate on large cities. My choice would be London and several day trips.

It’s easy to get around these days with cheap flights, so I don’t worry about distance much anymore. That said, don’t plan a flight or overnight train every two days, and don’t choose a spread out itinerary if the logistics are difficult At your age and with your interests I think Spain, and Barcelona in particular, would be a great choice.

I agree with augustin re: Barcelona. Spend 5 nights there and then catch a short 45 min flight to Ibiza for a couple days. Hit up Privelege on a Monday night to see Armin. Since you’re going to Tomorrowland these places will be right up your alley. Other cool places to sightsee then party for a couple guys your age would be a Prague & Budapest.

I suggest Spain. Must see is Barcelona, Madrid and Belchite, which is a wondreful absolutely empty town also known as a ghost town. It became so afetr Civil War, during Franco reign, Overall, its just great)in Spain you will have all kinds of vacation like sport, beach, culture and crazy wated parties.