This city is simply magnificent. The history, arts, architecture, the bosphorus strait, seagals, ferries, bazaars, shopping, carpets, bridges linking two continents, domes, minarets, food, cafes, weather, melancholy, soccer events with passionate fans all over the place, energy, people ... My wife and I are glad we visited and we are hoping to go again. This is one of the best locations to visit in Europe and in the world, offering the visitor everything.
I highly recommend Istanbul! I was there about 10 yrs ago (with a male friend) and we had a wonderful time. Didn't stay at the Marriott, which I'm sure is great... used points at the Hyatt Regency which was a fantastic hotel. There is so much to see in Istanbul. One day, we hired a private guide to show us the Blue Mosque, Hagia Sophia, etc. He left us at the Grand Bazaar to explore on our own. The rest of our time there, we explored on our own. It was a great time!
If you go, I suggest that you make time to venture down to Kusadasi for a couple of nights (no Marriotts here). You can hire a guide to take you to Ephesus; the last home of the Virgin Mary; and the Basilica of St. John. What an amazing adventure!
Also, in Antalya, there is a Ren resort that is gorgeous (used Marriott points here)! Had a suite on the top floor with a huge balcony... super! A great place to just relax and wind up your trip.
Here are a couple of links that you may find helpful...