Ferries, seagulls and the Bosphorus were my three main obsessions in Istanbul. Just taking a walk along the shores of the Bosphorus, watching the seagulls and then taking a ferry ride could get you in contact with more locals than any other activity. Although the Bosphorus is lined with multimillion dollar apartments and homes, the shores are utilized by people from all walks of life. You can see women with baby strollers, men fishing, families picnicking, as well as people exercising or walking in their latest name brand sneakers. It is a democratic place where everyone can come to enjoy nature, fresh air and the beauty of this wonderful city.
You can find the ferry schedule from this link.