If you are walking around the city, feeling hungry and don't want to indulge in a big meal, I would suggest going into any bufe for a snack or a light meal. Bufes are small snack shops that specialize in tost, Turkish style grilled cheese sandwich, and different types of sandwiches and hamburgers.
My favorite is the simple cift kasarli tost (two pieces of thick slices of bread with extra kasari cheese, grilled in a heavy iron grill) which goes really well with Ayran ( salty yogurt drink.) Ayran which is one of the most popular drinks in Turkey, is an acquired taste for most Americans. For those who are not adventurous enough to try a salty drink, there are also freshly squeezed fruit juices and Turkish tea that also goes really well with tost and other offerings at a bufe.
There are bufe chains all over town like Marmaris Bufe, Durak Bufe, Kristal Bufe, Baris Bufe, Taksim Bufe, Susurluk Tost ve Ayran evi which offer consistent great tastes for very reasonable prices. While some are small hole-in-the-wall type of places, some offer outdoor seating.
My most memorable tost and tea snacks have been the one's I have had on the vapurs (steam boats that carry commuters back and forth on the Bosphorus) or vapur iskelesi (ferry docks) and the make-shift cafe right on the water, in front of the Dolmabahce palace parking lot.