Hi Julie, congratulations on adding a few days in Istanbul. Tina in a posting called something like "2 or 3 day in Istanbul" got lots of responses, including mine. I have copied it here:
Hi Tina, I spent several weeks in Istanbul in 2016. One big change is the airport. Its' operational date for North American and Greek flights is still fluid. You won't be able to plan the overnight airport stay until you know which location will be used by your flights.
For your stay of 2-3 days in Istanbul I recommend the following in order to minimize travel time and maximize enjoyment. Purchase electronic visas, you may need multiple entry, depending on the overnight transiting procedure.
Buy RS Istanbul guide. Read thoroughly. Reread the sections that cover Sultanamet, what it offers, the walking tours that RS describes, and the interior descriptive guides that RS offers.
Hagia Sophia [at least 45 minutes] Topkapi [at least 2 hours] Archaeological Museum [at least 2 hours] , Turkish and Islamic Arts Museum [at least an hour]. You can easily double these times. I tripled them, but I have an intense interest in history, archaeology,architecture. These four places have individual admission prices or you can buy a Museum Pass which covers over a dozen major museums in Istanbul; you will need to calculate the economy and usefulness since you bypass the ticket booth and go direct to the entrance check station. If 2 days, maybe; if 3 full days, probably.
Underground Cistern [at least 30 minutes, not covered by Museum Pass]
Blue Mosque [at least 30 minutes], no admission but consider a donation, it is a Mosque in which Muslims worship, it is closed during "official" worship times, non Muslims are very welcome at other times. While visiting consider sitting on the floor and worshiping in silence which I believe will improve your appreciation of the blue tile artwork, and, indirectly , of Hagia Sophia, which was a spiritual worship space, both Christian and Muslim, for a millenia. BTW I understand it has been undergoing major restoration, so that might reduce the benefit of visiting it.
Hippodrome [30 minutes], it is outside so no admission price.