Been to Turkey for the last 4 years,
Wonderful place to stay, but never stayed in istanbul because it is hot there !!
I stayed in a city called Yalova, which is on the Asian side of Turkey, do visit the city and from there u can go to a nice village called Termal, which is located 20 minutes away between the mountains !!! if u get a chance to go and spend a day there, do it , u won't regret it !!they have hot springs and Turkish Baths!!
In Istanbul don't miss the Bayazeed Souk, u will need at least 3 days to cover it all !! so don't worry !! u will have alot of activities to do, yet u should plan well before going !!
I would be more than happy to help u plan ur trip to Turkey, and take my word, never go to a travels agent or tourists people, they will take u to certain places and u have to follow their schedule !! where as by checking few websites and asking few people, u can manage everything by urself and enjoy it !!!
Good luck !