Where to stay in Istanbul ?
Best place to stay in Istanbul old city part which call Sultanahmet district. Sometime Sultanahmet hotels could be expensive or full, than you can try Sirkeci, Gulhane, Beyazit or Laleli district. These places are still in walking destination.

What to see and do with a short time in Istanbul ?
Highly recommend to visit Blue Mosque, Hagia Sophia, Topkapi Palace, Grand Bazaar and Basilica Cistern. Dont forget to join a bosphorus cruise, Istanbul is only the place located on two continent in the world.

What to buy, what is special and diffrent ?
Istanbul is a center of handicraft shopping. World famous Turkish Carpets  and kilims , Iznik Ceramics, Turkish delight, Apple tea, Leather Jackets and bags, many kind of spice, etc.

What should I do in Istanbul at night ? Does it safe?
Istanbul nights are really colorfull and interesting.