Sus consejos:Figure out transportation and hotels prior to getting there. Istanbul Card is great and cheaper for using the tram if frequent travel is required. One can do a self tour but using a guide really adds perspective.
Stayed in Old Istanbul for one of my best vacations ever. Did a full day walking tour of Topkapi Palace, Hagia Sophia, Blue Mosque, Basilica Cistern, Hippodrome and Grand Bazaar - words cannot begin to describe it. Visited Spice and Arasta Bazaars as well. Lovely city overall.