Went to Belgium in 2014 for Tomorrowland! We walked all throughout Brussels sightseeing and trying their food. Belgium is known for their waffles, chocolate, mussels, and sausages, so a definite must get while you're there! I would recommend Neuhaus chocolate (they have locations everywhere). I found most of the restaurants through Yelp- the Yelp community just started over there so there isn't much, but it is useful in finding restaurants and random eateries! Some yelpers in Belgium who realized I was there were super nice and messaged me recommendations for when in Belgium, so I would find the active Yelpers and hit them up as well!
Tomorrowland was a crazy event; definitely a once in a lifetime experience. We got the flight package which included hotel, transportation, a surprise tour, tickets, food, and goodies! I would recommend doing the travel package if it is your first time; it is more pricey but it is worth the whole experience.