I was here in December 2018 and went on a free walking tour where they told us that you can buy tickets for tours inside the town hall for Wednesdays and Sundays. Tickets are only available on the same day as the tour and they're in English, French and Dutch (€7 for adults, €4 for children). You get them from the tourist information centre in the Grand Place.
You head through the archway and into the door on the right just before your allotted time. You can leave coats and bags downstairs, then head upstairs to have a tour of the various rooms. Our guide spoke brilliant English and showed us the rooms where the Brussels Berger-master and colleagues meet to determine what will happen in the city. You also see a ceremonial room where people can now get married. All in all, a good tour which lasts just under an hour.
Outside of course, the building is beautiful, but there are issues with it - it's not symmetrical for a start, the windows don't match etc. It's an awesome building none the less, but make sure you read up about it / do a tour to find out about the outside and its quirks.