One of the things I most enjoyed about Istanbul was the immense variety of fast food and street food. You never need to go to a restaurant in the city - the food will come to you.
In some cases quite literally. If you keep your eyes open in the area around the Grand Bazaar, around lunchtime, you can see waiters dashing across the street, carrying plates loaded with takeaway meals. Almost every courtyard has a little cubbyhole with a chai (tea) maker, and waiters scurry about carrying glasses of tea on little metal trays, or scouting for orders from the market stalls.
"Simit! Simit!" - that cry is almost as typical of Istanbul's noisescape as the call of the muezzin. A sesame seed covered soft bread ring, simit is the commonest of the snacks sold in the streets of the city. Some simit sellers have a sort of spear with the simit stacked on it. Others carry a pile of simit on a tray on their heads. Some have gone for a modern solution - a little barrow on bicycle wheels.
Borek is another of the fast foods. In cafes, you'll see huge wheels of cheese borek, but in the street, you'll find 'cigara borek', little sticks of borek. I actually prefer these - they're crispier and less fatty.
You'll find roasting chestnuts in some places. One guy does a roaring trade near the bus stands at Eminonu. Elsewhere, you can find corn on the cob slowly roasting. Most vendors stick to a single food - but there is one stall near Ayasofya that does both of these.
A sweet tooth is easily satisfied. Fried dough strips soaked in sweet syrup cost a lira (around a dollar). You'll soon learn to suck the syrup as you bite, or risk the syrup running down your chin. Or try gozleme, pancakes with sugar (they come in savoury versions too).
Thirsty? Stop at one of the barrows loaded with oranges, pomegranates, and a stainless steel squeezer, and you can get a glass of juice squeezed while you wait.
Doner kebab and fish sandwiches (balik ekmek, or 'fish bread') are not street food - they're a little higher on the evolutionary ladder. But you can order at a window and take them to eat elsewhere. And like the street food they're cheap; we paid 3 lira on average.
But my best memory of Turkish street food is not from Istanbul - it's from Buyuk Ada, the island we visited on a bright spring day. Right by the ferry pier is an ice cream parlour with an amazingly varied selection of ice cream ('dondurma'). Chestnut icecream, sour cherry ice cream, pistachio, coffee and chocolate chip. Icecream is never cheap in Turkey - we paid more for our icecreams than we did for most meals. But it was well worth it. If you think the Italians know all about ice cream... this parlour certainly is up there together with the one just off the main piazza in Siena, or the one on the Tiber Island in Rome, as far as I'm concerned.