+1 to all of these! Istanbul is ENORMOUS so you won't be able to hit close to everything. But definitely go to Sultanahmet (many things are clustered together near there, like Aya Sofiya, Blue Mosque, Topkapi Palace, Grand Bazaar, etc) for at least 1 day, if not 2. You could frankly spend your whole time there and not see everything but I also encourage you to get outside of that area to have fun, like to Taksim Square (shopping, food, walking) and Bebek (high end shopping, food, walking, can also take a look around Bogazici University, which is beautiful).

I'd also highly recommend doing a ferry up the Bosphorus to the Black Sea. It doesn't take all that long and you'll see so much of the city from the water that you won't see while being in the middle of it. It's just such a stunning place, perhaps the most beautiful city in the world-- but you don't really get that perspective until you step a bit outside. You'll see incredible mansions and mosques right along the water. You can get off the ferry really at any little town to have a great lunch and then either get back on or take a bus back. Public transportation is easy and fantastic.

I recommend Culinary Backstreets for a food tour.

Yes to the above. Also do a food tour, the new modern art museum, a boat trip to the princess islands, walking and people watching in neighborhoods like ortakoy and of course taksim.

Love Istanbul! It’s been many years but some things I remember fondly - The Cisterns, Blue Mosque, Haghia Sophia, the big spice market, Bosphorus Cruise, a Turkish bath

Female solo traveller