There are many places on the list not to miss.
The Basilica Cistern, Süleymaniye Camii, the Topkapi Saray complex, Ayasofya... and so much more. The city has been called the world's largest archeological museum.
Whatever you want, someone has it for sale. Probably you don't even know you need it, but they will have several varieties to suit any taste or pain threshold. There are many things hidden and many layers to peel back.
The best thing is drinking tea beside the Bosphorus, watching the life go on.
How many days do you have?