In Istanbul there are many cruise deals for the Bosphorus canal. If you decide to take one I recommend a night cruise - all lit up and beautiful. This photo was taken on one of those cruises, it's the Bosphorus Bridge with colorful lights that change every so often, the contrast with the darkness of the night is lovely. If you decide on a cruise, I recommend you go at night.
Taking the ferry from Eminönü to Üsküdar (on the Asian side, just in front of the Golden Horn), walk south, with the sea always on your right, passing a small mosque and what looks like an air traffic control tower, you will find one of the most wonderful places. A beach bar where you can sit and relax by the water on comfortable cushions along the shore. And you can appreciate the magnificent sunset from there, with the sun setting behind the Topkapi Palace, Haya Sophia, Blue Mosque and Galata Tower, a beautiful view! If you go to Istanbul, I recommend that you go there at sunset.
It's worth missing your ferry and to wait (with the Turks) for the next one (about half an hour) as all the "station" smells like fish and vegetables fresh. The Turks sit on tiny chairs savoring freshly caught fish from the Bosphorus. There's a seller of threads and corncobs, for the sweet tooth. Sitting on the steps of a sort of makeshift amphitheater is a good time to open food cans if you don't have much budget like sarma (spicy rice wrapped in palm leaves), beans with peppers, tuna, some tomatoes - Turkish food is healthy and delicious even if you eat it from cans. All the Turks and tourists look to the Golden Horn, waiting for the ferry, a friend, or for something to happen. It's worth missing your ferry...
This summer we went on a trip to Turkey for 15 days, and one of the fondest memories that I have, accustomed to traveling on my own, is a day that we went around on our own in Antalya. There was a little beach with sunbeds and a bar to drink where the prices of the drinks were printed on the shirts of the bartenders (they were not expensive), and a historic center crowned with a beautiful medieval harbor offering stunning views of mountains that vanished and appeared in the day. It is also advisable to go around on one of the boats that constantly comes out of there.