I worked here for a few months some years ago, though I didn't see the centre so much (I think it's better now where I was in the east but I would take a taxi, train and boat = up to 2 hours!), managed to see the main places apart from the Roman Cistern, which is also a good place to escape the heat. Did you see all the cats too 😺 Balik ekmek, always nice to have that fish sandwich. I enjoyed getting the boat across from the Asia side where I was staying to the Europe side, and Haydarpasa Train Station is always work a look, on that side. And Baghdad Street is quite good on that side, not so touristy and I think it's developed a lot since I was there. And finally Büyükada island is pretty interesting, good you went there too.