Istanbul is one of my favorite cites and I'm so glad that Viking is going there. It's been several years now since the cruise ships have visited and it's such an interesting, vibrant port. You are spoiled for choice as to what to see! You've already gotten some good advice about that and only you know what interests you most. The only suggestions I would give you is to wear comfortable shoes as it's very hilly and whatever else, don't leave Istanbul without eating at a restaurant. Or two or three. Turkish food is delicious and service is gracious. Oh, and if you want a souvenir, on the low end of the scale the pottery is gorgeous and on the high end, textiles, including rugs of course, also are well worth considering.