The historic areas of Istanbul are great to visit. The Aya Sofya and the Blue Mosque are obviously the center pieces, but there is more in the area; the underground cisterns, Topkapi and Dolmabahce palaces, the Chora Church. All these are nice to go to except perhaps for Topkapi, which I felt was overpriced and over-rated (The Harem is not at all worth it!). The Bosphorous is always there, and wonderfully accents all of these places. Give Istanbul at least 3 days!