your welcome i live in istanbul so i know a litlle but its too hard to explain because in istanbul everything changes nearly ever month. prices,, places...
you can find nearly every where in istanbul. but i like "kırçiçeği" restaurant's where is placed in ortaköy. probably i will go that side of istanbul. Add: mecidiye mh. ortaköy değirmenci sok. no:41. (http://www.kircicegi.com.tr/subeler.htm)
namlı is the name of the place : http://www.namligida.com.tr/namligurme/ (it is a little bit turistic and expensive) next to namlı there is a güllüoğlu ( they are not like the others i think you can eat also "börek" in güllüoğlu ( something with cheese or spinach / its salty)
near karaköy there is a street . it is becoming popular. you can find some coffe places. tehy are near mumhane street (caddesi) (for example: karabatak ...)
if you go to a place that can you eat MEZE (like tapas) we eat them with drinks (for example: rakı (thats turkish too)
i really like vegetable meze's like : zeytinyağlı sarma, enginar, mercimek köftesi, şakşuka, patlıcan ezme, barbunya... ) you have to ask for it :/
for princess islands in Büyükada : there is a church top of the island. and you can drink something and watch istanbul if the day is sunny
these are all touristic information may be but it can help you may be ....