Is it worth taking one day in Istanbul to take a cruise to these Islands. We have 5 full sightseeing days in Istanbul.
Given the short time you have and the incredible number of attractions in Istanbul, I would play this by ear and see how you feel (you don't need to decide in advance). The Princes' Islands are a wonderful getaway to somewhere so much calmer and mellower (and just different). I really enjoyed my trip and I hopped off at Buyakada (the big island) as well as a smaller one whose name I can't remember. In Buyakada, I took one of the horse buggy carriages and rode around the island, and then just walked around and ate and enjoyed myself. It was worthwhile but I was in Istanbul for over two weeks so I felt like it was worth getting away to a less intense location. If you've been to Greece you may not be impressed - I have not, so I really enjoyed these islands.