Some great suggestions - I just love Istanbul - Sultanahmet for the wonderful views from every direction - the Aghia Sophia/Blue Mosque/Topkapi (the Topkapi harem is a must) and so much more - touristy yes but the views from every angle forgive all that - for nightlife - great bars/restaurants are Mozaic and Shah bar
Istikal Cadesi is completely different a hive of activity, the Galata Tower and Pera Palas hotel are nearby amd are a must see and like a step back in time. So many great Turkish restaurants to choose from but if you want a 5 star treat for half the UK price go to Changa (kust off Taksim square) - fusion cuisine at itd best - rated one of the top 50 restaurants in the world in 2003 - so good we went twice
The Gand Bazaar is a must see even is you dont end up buying - I can recommend the Donna Kebaps or the Fez Cafe for sage tea and the best carrot cake in the world (a got the tip from the Guardian restaurant critic)
Enjoy Istanbul, savour the atmosphere - I guarantee you will want to return