My vote would be for Emirgan Sutis. You can both try examples of the local cuisine and watch the ships going back and forth through Bosphorus. Transportation is also available from different spots, such as Taksim and Besiktas.
You can take public buses going to Sariyer. Now that you want to have brunch, then I would strongly recommend you to have the traditional Turkish breakfast first, with the traditional black Turkish tea for sure. Here is the link of the webpage where you can see the location and menu of the place:
P.S. The place is much quieter during weekdays. You might choose an alternative for the weekends if you like tranquility.