Food&Wine, a famous magazine in the USA, made a list of ‘world’s best cities for street food’.
There are 24 cities in the list and Istanbul is the fifth best city for street food in the list. Food&Wine reports that doner kebab is a well known street food in Istanbul, but it is not the only one. It says “But the capital of Turkey offers many more quick foods, with specialty kiosks scattered throughout the city hawking börek (flaky pastry), simit (ring-shaped sesame bread that recalls a pretzel) and kumpir (roasted potatoes stuffed with anything from ketchup and pickles to olives and sausage).” (resource: Food&Wine)
Note: The magazine shows Istanbul as the capital of Turkey, but it is not. The capital of Turkey is Ankara.
You can read more on simit here.
You can see a picture of kumpir here.