You have to see the blue mosque and the Hagia Sophia both buildings are amazing. Another touristy type site that is a little less known is the cistern that is under the city. I thought it was cool how they had reused stones from other buildings to make the columns. There is also a huge market you should check out. It is covered, but not heated or anything. Be prepared to do some hard bargining and have a lot of people try to get you to come into their shop, but there is some good stuff. Let me know if you have any questions. Have fun!!