I lived in Istanbul two years (ten years ago) and had previously visited many times. For some places, Hagia Sophia as DrZ mentioned, Topkapi palace, the Blue Mosque, etc., you might have a tripod or flash restriction or have to pay a small fee to use a camera. Those are the only restrictions I know about, but the rules may have changed over time. Other than that, street and cityscape shooting is a lot of fun and unrestricted. You will probably get requests to provide a photo when people are in the image. If someone who doesn't want to be photographed waves you off, you need to be respectful of that. I'd leave the Nikon and zoomer at home unless you want to bring it as a backup to the CLE and leave it in your hotel room.
The hottest tourist sites are around Hagia Sophia, and include Topkapi Palace, the Blue Mosque (Sultan Ahmet Camii), and the ancient underground cisterns (Yerebatan Sarinci). You can cover these 4 sites in one day. Dolmabahce Palace is worth a look also -- but it was all guided tours, about an hour or so per tour. There's good street shooting and shopping around Taksim Square and Istiklal Caddesi (along the tram lines). A bit off the beaten path is Kumkapi, an area with small cozy restaurants and roaming troubadours. I used to dine there with friends at Talep'in Yeri or Kor Agop. March may be a bit early, but they used to use ferry boats for tours up and down the Bosporus. The tour price was under $5, as I recall. If the tours are not running yet, you can still take ferry boats back and forth across the Bosporus to get some good scenery and shots. The Grand Bazaar is a lot of fun for shopping and photography as well. Also, the spice bazaar (Misir Carsisi or Egypt Bazaar) is interesting and fun to visit. Have fun, and feel free to ask any more questions.
. But I'm sure our Turkish members can easily point you in the best directions.
), midway of Istiklal Street. Alex Webb's recent "Istanbul" book is also great stuff.