There may be no modern city on Earth with a cultural lineage as rich and varied as Istanbul's. Founded in 657 BC as the Greek colony of Byzantium, it later became the capital of the vast Roman Empire under the name Constantinople and gained distinction as the largest and wealthiest city in the European world. After over a thousand years of holding its walls against invaders, the capital fell to the Ottomans, where it served as the heart of their own empire for nearly five hundred more years until the nation of Turkey was born from the War of Independence in 1923. And yet Istanbul has continued to outgrow its natural water barriers, it's borders stretching beyond the Bosphorus River that not only splits the city, but the two divergent worlds of Europe and Asia. Today, 17 million people call Istanbul home, playing out their lives among the ancient minarets and mosques of the old city of Sultanahmet, the shopping streets and vintage monuments of Beyoğlu, the promenades and waterfront cafes along the Bosphorus, and the steel skyscrapers of Maslak. Tourists will abound, but that only goes to prove that there is much here to intrigue a culturally-minded world traveler.
An international city that truly wears its East-meets-West heritage on its sleeve, you'll not find many modern Turks in fezzes or hamams (Turkish baths) used with the same frequency as during their Ottoman prime, but you'll still discover traditional hookah bars and doner kebob shops sharing this city with the types of Western consumer brands that line Istiklal Avenue. And while the large majority of Istanbulites are Muslim, there is a historic tolerance toward non-Muslims, a fact proven by the Sufi practice of Whirling Dervishes. To catch one of these performances is a unique spiritual experience, while Islamic landmarks like the Blue Mosque and the Hagia Sophia are essential items in every itinerary no matter your religious persuasion. Although the usual tourist rarely strays from Sultanahmet unless it's via cruise ship on the Bosphorus, you'll want to move out along the Golden Horn or into the neighborhoods that look out on both sides of the river if you seek a better mixture of "authentic" Istanbul and the local restaurants and nightlife that you'll invariably be aiming to enjoy.
If you're looking to skip out from the urban melee, the Princes' Islands are a more peaceful alternative in the Sea of Marmara, with their yellow Mimosa blooms, horse-drawn transport, and white-wooded structures. The beaches on the Black Sea to the north are bit cooler, which shouldn't deter you from visiting destinations like Şile and Kilyos for the swimming and water sports, while nearby Ağva may just have some of the most hike-able coastline in the north of Turkey. Sadly, beyond Belgrad Forest, which is a refreshing natural reprieve for hikers and bikers, there aren't a ton of actively adventurous options within immediate proximity to the city. If you're willing to make a full day of it, there are a few waterfall hikes, endless cave systems, and much more to pique your interest, just don't expect them at every turn.
Turkey is a country of romantic notions and Istanbul, despite being lifted of its official "capital" status nearly a hundred years ago, is both the historic and contemporary heart of the nation. Goosebumps will raise from the trill of the evening call to prayer, your heart will race in the bustle of these ancient boulevards, your limbs will succumb to the heated marble and foaming sponges of traditional hamams, and your taste buds will revel in the delights of doner, kumpir, and baklava. Consider your participation a done deal.