We enjoyed it. There’s a tour of the EU Parliament in English once a week by someone who works there, that was great, I can’t remember if it’s free but if not it was cheap. There’s also the Parliamentarium interactive centre for the history. The House of Europe less good (can you tell we took a teenager mad about politics).
We also went to the Natural History Museum, the comic strip museum and the Magritte. Lots we didn’t get to that others have mentioned.
Walking a lot we found we kept going in and out of pretty, historic areas and contemporary working areas, but that’s what you get in any big city.
Food was great, both traditional and international from Belgian colonial history meaning there’s some great African places.
Very easy travel on the Eurostar and the Brussels metro. We stayed in a gorgeous little town house with garden, which I would highly recommend (on Booking.com).