Buy the food somewhere cheap.
Eat it somewhere good.
Restaurants near the big sights are expensive and mostly bad. A few streets away there is always a bakery, a market stall or a counter where locals actually eat — and within ten minutes' walk, a park, a canal bank or a wall with a view. This site pairs the two, city by city.
London
United Kingdom
Bernie Spain Gardens, Granary Square and the Regent's Canal steps, and 3 more
15 places to buy lunch
Paris
France
Place des Vosges, Canal Saint-Martin, Quai de Valmy, and 3 more
11 places to buy lunch
Rome
Italy
Giardino degli Aranci, Passeggiata del Gianicolo, and 3 more
11 places to buy lunch
Vienna
Austria
Burggarten, Donaukanal at Schwedenplatz, and 3 more
12 places to buy lunch
Chicago
United States
Chicago Riverwalk, Milton Lee Olive Park, and 3 more
10 places to buy lunch
How the picks are made
Somewhere you're allowed to sit
Grass, steps, a wall, a bench. Not the Spanish Steps, where you'll be fined, and not a park that locks its gates before you arrive.
Food that survives the walk
Sandwiches, pizza by the slice, boxed noodles, things wrapped in paper. Anything that needs a fork and a flat surface is out.
Independent and cheap
No international chains. Mostly single-site shops, market stalls and counters, at prices locals pay for lunch.
Close enough to matter
Walking times are measured from the shop door to the spot. Most are under fifteen minutes; the few that aren't say so.