Antwerp gives you four overlapping cities to walk through — Rubens's Baroque Catholic city around the Cathedral and the Grote Markt; the merchants' Renaissance city of the Plantin-Moretus workshop and the guild houses; the modern docks city around the MAS, the Red Star Line Museum, and the new Eilandje quarter; and the fashion city around MoMu and the Royal Academy.
For a single day, the priority circuit runs: Cathedral, Grote Markt, lunch on Vrijdagmarkt, Plantin-Moretus, walk along the Scheldt to the MAS for the free roof terrace at sunset. For two days, add the KMSKA and M HKA in Het Zuid, the Rubenshuis, and an evening in Zurenborg. Three days lets you add the Diamond District, a tram ride to Cogels-Osylei, and a beer tour at De Koninck.
The most local way to spend an afternoon is "een terraske doen" — sitting on a café terrace on the Grote Markt or Groenplaats with a Bolleke, watching the city pass.