Sablon Antiques Night
Twice a year the Sablon dealers stay open past midnight, put a table on the pavement and let people handle things they would normally have to ask about.
The trade comes for the first two hours and everybody else for the rest. Prices are not lower, but the conversations are longer, which is the actual point.
The specialisms are narrower than the square lets on: one dealer sells nothing but scientific instruments, another only textiles from before 1900, a third works exclusively in Belgian silver and will tell you at length why that is a category.
Four restaurants on the square run a single-course menu at a fixed price so the queues move. The chocolate shops stay open too, on the grounds that they always do.
The square closes to cars from six. The nearest parking is under the Palais de Justice, uphill and worth the walk.