The former Constantinople, Istanbul is one of my favorite cities on the Planet.
Existing in Europe and Asia, it is rich in history, culture and so, so good food.
A few things to note: US citizens need to get a Visa. It is easy to do online. Currently it costs $20. http://www.mfa.gov.tr/visa-information-for-foreigners.en.mfa
Where to stay -
If you stay in the historic district, all the major sites are easily reached by walking.
On the Do not miss
The Basilica Cistern
[For a fun introduction, watch the movie "Inferno" before your trip]
An added bonus is the pleasant temperature when you venture inside the Cistern.
Medusa stones in the Cistern
Topkapi Palace, The Blue Mosque, and Hagia Sophia are also close by. Do not Miss!
: Women need to cover their hair & shoulders. Tourists wearing shorts should have knees covered. The Blue Mosque is an active mosque, so be respectful. Luckily, it's such a tourist attraction, that they offer a cover-up for men & women to use at the entrance. Also, you can buy scarves everywhere in Istanbul. Bonus souvenir to bring home!
Inside Hagia Sophia
A visit to Istanbul should include a visit to the Grand Bazaar. Watch your wallet, be prepared to negotiate and walk away (even a few times), but DO enter this amazing maze of shopping delight. There are deals to be had and some nice vendors. Patience and knowing your spending budget makes it a more pleasant experience.
Lastly, do enjoy some amazing Turkish cuisine. Dark chocolate pistachio baklava and organic thin Turkish Delight were my favorites. Every meal was amazing! Lots of vegetarian options, great soups, breads, and kebabs for the carnivores.