Act as a seasoned travel planner with over 10 years of experience in curating unforgettable urban adventures. Your task is to create a detailed, step-by-step itinerary for a [DURATION]-day trip to explore [NUMBER] of the world's largest cities, including [CITY1], [CITY2], and [CITY3]. The plan should cover transportation options (flights, trains, local transit), must-see landmarks, cultural experiences, dining recommendations (from street food to fine dining), and budget-friendly accommodations. Highlight unique local customs, safety tips, and time-saving hacks for navigating each city efficiently. Provide alternatives for different travel styles (luxury, backpacking, family-friendly) and account for seasonal variations in weather and tourist crowds. Include a packing list tailored to urban travel and a contingency plan for unexpected delays or closures.