Half of Istanbul's food culture happens standing up. All of the following are within walking distance of us and each will do instead of a meal.
Simit. Sesame, ring-shaped, fresh. Sold warm from carts in the morning; by the afternoon what is left has hardened, so buy early. White cheese or tea alongside. A simit costs about a quarter of a coffee.
Balık ekmek — the fish sandwich. At Eminönü, beside the bridge. Grilled fish in bread with onion and lemon. Touristic but real; prices are fixed and posted. The right lunch on the day you cross the bridge.
Kokoreç. Not for everybody, but it is Istanbul's signature. Places in Beyoğlu stay open past midnight; a portion or in bread, and they will ask about chilli. If it is your first time, ask for a half.
Stuffed mussels. From street trays, with lemon. Freshness is everything: a busy tray is a good tray, because the stock turns over. In summer heat, avoid any that have been sitting out through the afternoon.
Chestnuts and corn. Chestnuts in winter, boiled corn in summer. Good for warming your hands, not for filling you up.
Islak burger. In Taksim, late. A garlicky, steam-softened burger under glass. Nobody claims it is gourmet; at three in the morning it is exactly what you want.
On prices: ask, even for street food. A stall with no posted price can charge double in a tourist area.