One of the most popular excursions in Istanbul is a ferry ride to Anadolu Kavağı, near the entrance to the Black Sea. The Bosphorus Cruise offered by the city-run Şehir Hatları company costs just 15 Turkish Lira, making for a cheap and easy day out on the water.
It was the mid 15th-century, and although the Ottoman army had long since surrounded the city, Constantinople was proving stubbornly resistant. In order to more effectively isolate the Byzantine capital, the invaders hastily constructed the Rumeli Hisarı. This fortress along the Bosphorus is still in marvelous condition, and makes for a fun outing.