Istanbul spans two continents, and this shortlist keeps a brief stay focused on what earns its place. Start in Sultanahmet, where the Hagia Sophia and the Blue Mosque face each other across a square, and the underground Basilica Cistern reveals Byzantine engineering few visitors expect. The Topkapi Palace fills in Ottoman history with sultans' quarters and treasury rooms. Cross to Beyoğlu and walk Istiklal Avenue toward the Grand Bazaar for shopping and street life, then use an Istanbulkart to hop a Bosphorus ferry — ideally timed as a sunset cruise — for skyline views connecting old and new Istanbul in one frame.