Concierge.com's insider take:
Catch the Friday morning spectacle (8 a.m.–1 p.m.) at Vrijdagmarkt, the square in front of Museum Plantin-Moretus, where a spirited auction of furniture, household goods, and secondhand clothing has taken place every week since the 16th century. Sales are conducted in Flemish but the auctioneer will accept bids in English. On the cobbled lane around the Cathedral of Our Lady, the Saturday Lijnjwaadmarkt (9 a.m.–5 p.m.) is the place for good buys on antiques, small collectibles, and bric-a-brac from Easter through September. Every Sunday on the square at Sint-Jansvliet (9 a.m.–5 p.m.) there's a small but wonderful little antique market with low prices and sellers still happy to haggle.