"What about Poland?!?"

It is really beautiful and you can see the old and the new buildings and how the city has changed over time.  I would recommend reading "Istanbul" by Orhan Pamuk before you go, it is an interesting look at the city.

Taksim is a great place for eating, shopping etc.  Then of course their are the tour guide favorites, the Haggia Sophia, the Blue Mosque.  I really enjoyed Topkopi palace.  The grand bazaar is fun but very overpriced.  Make sure you try gozleme, and Lahmajun.  Actually, all the food is delicious.  

There is so much to do, you really can't go wrong.  I have found Turks to be very helpful and very warm.

for all the great comments in this diary.  Enjoy your trip!