Cruise stop

Like other large, old, and historically important cities, Istanbul has many days of "must sees," and everyone will prioritize them differently. Here is Rick's opinion about the most worthwhile sights (click "At A Glance"): http://www.ricksteves.com/europe/turkey/istanbul

Now you need to read more about each one, and decide what to prioritize for your interests. For me, the highlight of highlights was the Aya Sofia; for my mother, it was the Mosque of Süleyman the Magnificent.

Do lots of research and then decide what hold the most interest for you. The excursions offered by the cruise lines are expensive, but they do save a lot of time, logistical headaches, and may very well cover the things you want to see and do. In addition, if the excursion is late the ship will wait for you -- not so if you are doing things on your own.

I answered this very question about two or three weeks ago, but darned if I can find it. Maybe you, or someone more knowledgeable on how to surf the site, can find it.

I agree with TC.....with that little time, I would definitely sign up for a shore excursion thru your cruiseline. that way you can pick the most extensive one that hits most of the highlights, and if you can jump lines, their local tour provider will probably figure out a way to make that happen.
There are times to save a buck, but then there are times to maximize time (and likely they will help you do that without worry about getting back to the ship on time).
Istanbul is fabulous....you will love it.

Please disregard my previous post. It is erroneous.

If you have not been to Istanbul before, I'd suggest you head over to the the Aya Sophia, Blue Mosque and Topkopi Palace. They are all right next to each other. A decent visit to all three will take more than 7 hours, but you could take a taxi to get to/from the cruise port and give them a more hurried visit. There is a small bazaar just down the hill from the Blue Mosque (Arasta Bazaar) and restaurants in most directions nearby.

Good luck with whatever you decide to do. There are so many interesting things to see and do in Istanbul that its hard to make a bad choice.