From London we are taking a day trip into Brussels via the train, or is the Eurostar? Because it is a one day outing, what do you recommend as the most important places to see?
Well the Manneken Pis because it's cute. I would visit the Grand Place - there are some very beautiful buildings here. Then grab some waffles, frites with mayo, and a cherry beer. Oh don't forget to stop at Leonidas for some amazing chocolates!
Near the Grand Place is the world-class National Art Museum. It just finished a major renovation and is worth a visit.