The Bosphorus Strait divides this amazing historical city into the European and the Asian side. Istanbul is an architect's paradise! It is known for its Byzantine and Ottoman architecture, but its buildings reflect the various peoples and empires that have previously ruled the city primarily the Romans.
I had a wonderful time in Istanbul. Even 3 days in the city seemed less...One should definitely visit the famous Hagia Sophia, the Blue Mosque, the Topkapi Palace. I suggest do take a full one day pass in which you can cover all of these places. Also if you individually take passes for each of these places it turns out to be very expensive and the queue very very long..Also, remember that most of these places close at 5 / 6 PM. So if you want to cover all in a day, its better to be early.
Another Must do is the Bosphorus cruise both by the day and the night! If you are the romantic sorts, do take the dinner cruise offered by various tour operators. Make sure to book well in advance.
Do not miss the Grand Bazaar...it has about 60 streets and over 5000 shops. Do remember which gate you have entered from or else prepared to get lost...its huge, its fascinating and its GRAND! You can shop for ceramics, carpets, silver jewellery, copper ware and various other antiques..Ladies, make use of your natural qualities and BARGAIN! One trick that never fails is to show interest first, then seem not so interested and walk away! you will definitely get the price you bargained for! Do not leave the bazaar without having a Turkish coffee at one of the various cafes!
Very close to Grand Bazaar is the Egyptian spice market...Get ready to be overpowered by the aromatic smells of various spices and condiments! Do take some and savor back home!
A trip to Istanbul is incomplete without some Belly Dancing! Sultana's 1001 Nights is one of the most touristy places for dinner and belly dancing shows. The concierge at my hotel recommended the Orient House and Kerwansaray for Belly Dancing. You can try these out too!