Istanbul is a magical city spanning two continents, where the East meets the West. From the architectural wonders of the Hagia Sofia, to the bustling fish markets of Kadiköy, and the hip coffee shops of Karaköy where young people flock on the weekends, there's something for everybody.
Start your journey by taking a tram (suggested during rush hour; 5 lira, 1 hour) or taxi (~30 lira, 20 minutes) from Ataturk International Airport to the Sultanahmet neighborhood. Here you can tour the Hagia Sofia, a former Greek church, turned Ottoman mosque, turned museum.
Sultan Ahmed Mosque also known as the Blue Mosque is right across the square. Make sure to bring a headscarf, shirt covering your arms and pants that cover your knees or you'll get stuck wearing one of the dusty, public chadors!
Skip the overpriced restaurants in Sultanahmet that cater to yabancı (the Turkish term for "foreigners"), and head down to the Eminönü ferry port where you can float over to the Asian side of the city in just 20 minutes. The ferry is a form of public transit that thousands of Istanbulites use every day and also one of the best spots to view the city's "skyline".
Once you arrive at the Kadikoy limani, it's just a five minute walk to the Moda neighborhood where you'll find everything under the sun. Walk down Güneşli Bahçe Sokak where you'll be lured into one of the many fish restaurants by the persuasive (and handsome) hosts. If your sweet tooth is starting to kick in, pop into Beyaz Fırın Kadıköy or Sekerci Cafer Erol for traditional Turkish sweets including baklava, cookies, and marzipan.
If you do decide to stay the night on the Asian side, Hush Hostels has two locations: one is right on the main street in Moda and another about a 15 minute walk away in a quieter part of town (called Hush Hostel Lounge). Shared dorm rooms are $12 and private rooms are just $20.
Spend the last hour of daylight at the Moda Seaside, a long path along the Bosphorus full of couples canoodling while they watch the sunset and colorful balloon dart games set up on the rocky edge of the path.
As dinnertime approaches, make your way to Cibalikapı Balıkçısı where you can start off with a variety of Turkish mezes such as stuffed grape leaves ('dolma', in Turkish), yogurt sauce, octopus, and sea greens. But, don't stuff yourself too much and save room for a fish entree!
If you haven't fallen asleep after this, enjoy a keyif, as the Turks say with a nargile (hookah pipe) back in Moda at one of the many bars on the main street.
Before heading back to the airport for your onward journey, spend a few hours getting lost around the European side of the city. Beyoglu is the perfect neighborhood to start in, but make sure to wear good shoes, as you'll be climbing some steep hills! Here you'll see what typical Turkish life is like with grandmas out shopping for dinner ingredients, men sitting in the local cay house, sipping tea and playing backgammon all day, and children playing in the streets. If you have time, head over to the Galata Tower and take the elevator up to the top for grand views of the entire city.
Make sure to go coffee shop hopping, as there seems to be an infinite amount of cafes to stop in. End the day with lunch and a Türk kahvesi in Karaköy, down by the water. Despite how many of them are condensed into just a few blocks, each and every one seems to be busy all the time. If you do decide you want to stay here for the night, Portus Hotel (~$36 for a private double) is a small hotel, with simple, clean design right in the center of the neighborhood.
Bonus day: If you fall in love and decide to stay longer (which you probably will!), take a trip out to the Princes Islands. In the summertime it's packed with folks from the city looking for a place to cool off on the beaches, but early on in the season you'll find a way to escape the masses if you venture out a bit. Büyükada is the big island and the busiest, but walk 40 minutes up the path to the Dilburnu Milli Park and you'll find lovely spots to picnic under the pine forest as well as views of the Sea of Marmara that will make you think you're at the end of the world.
Turkish Airlines flies non-stop from multiple US cities including New York, San Francisco, Los Angeles, and Atlanta. You can make a stopover for 24-36 hours before continuing onto one of their 272 destinations worldwide.