Lonely Planet review
One of the most enjoyable, and certainly most romantic, night-time activities in İstanbul is to take a Bosphorus ferry. Enjoy the view back to the Old City, the twinkling lights, the fishing boats bobbing on the waves and the powerful searchlights of the ferries sweeping the sea lanes.
The best ferry to catch for this purpose is the one from Karaköy (just over the Galata Bridge from Eminönü) to Kadıköy. Just go to Karaköy, buy two tokens (for the voyages out and back) and walk on board. When you reach Kadıköy you could head into the backstreets and grab a bite to eat.
A shorter ride is the one from Eminönü to Üsküdar. When you alight in Üsküdar, you could have a delicious feed at Kanaat Lokantası, or turn right and walk around the coast to the çay bahçesis (tea gardens) near the Şemsi Paşa Camii (Şemsi Paşa Mosque). After a tea or two, continue on the popular waterside promenade past the famous Kız Kulesı (Maiden's Tower) - a gorgeous walk on a summer's evening.