I spent a few days in Istanbul in June and really enjoyed the bosphorous strait, as you can see from the pics lol. Karakoy Pier and Galataport were such a delight, just watching the seagulls flying around in flocks and eating fresh grilled fish sandwiches. There's just something very calming and special about this part of Istanbul.
I'd also highly recommend the Istanbul Museum of Painting and Sculpture in Galataport, underrated and beautifully curated. You could easily spend a couple of hours in there.