I really liked Istanbul! Colorful city! They also say about Istanbul: "City of contrasts". We lived in the historical center. There was a restaurant on the roof of our hotel and in the evening a beautiful view of the Bosphorus and the Blue Mosque!
You can get from the airport to the main station on the S8 branch, pass 13 stops at the 14th stop, get off, The ride takes about 35 minutes. A ticket was bought at the box office and immediately validated for 21.