Carlsplatz is Düsseldorf's central market, close enough to the Altstadt to fold into any walk through the old centre. The stalls sell fruit, vegetables, fish, meat, cheese, flowers, sweets, and plenty of ready-to-eat food, so it works as both a practical market and an easy lunch stop.
It is most enjoyable around midday, when office workers, shoppers, and visitors overlap under the canopies. Come hungry, graze rather than commit, and use it as a softer counterpoint to the beer halls and shopping streets nearby.