Istanbul is a very interesting city. It is both modern and historic. We spent about one day there and were able to see several of the renowned sites just by walking around the Sultanahmet area. There are so many cool things to see there—my favorite place was probably the Grand Bazaar. The highlight of that trip for me though was definitely eating doner every few hours.
Grand church, then mosque; now museum & symbol of Turkey
17th century blue-domed structure; also known as Blue Mosque
One of the largest and oldest covered markets in the world
Ancient underground water tank below the modern city
Gigantic 15th century Ottoman palace that is now a museum