If you have just a few hours to spare in these incredible destinations, there are still plenty of opportunities to explore and experience the local culture and attractions. In Hanoi, Vietnam, take a stroll around the Old Quarter and visit the Ngoc Son Temple and the Hoan Kiem Lake. In Belgrade, Serbia, make sure to visit the Kalemegdan Fortress and enjoy the vibrant atmosphere of the Skadarlija district. Meanwhile, in Luxembourg City, Luxembourg, be sure to wander around the charming old town and visit the impressive Bock Casemates.

When you have a few days to spare, extend your exploration to Sofia, Bulgaria, and witness the stunning Alexander Nevsky Cathedral and the historic Boyana Church. In Reykjavik, Iceland, don't miss the chance to visit the Blue Lagoon and take a tour of the Golden Circle to see the majestic Gullfoss waterfall and the Geysir geothermal area. In Tulum, Mexico, spend your days lounging on the beautiful beaches or exploring the ancient Mayan ruins overlooking the Caribbean Sea.

For a week-long trip, the possibilities are endless. Spend your time in Florence, Italy, admiring the artwork at the Uffizi Gallery and the majestic Florence Cathedral. In Petra, Jordan, embark on an unforgettable journey to the ancient city carved into the rose-red cliffs, and make sure to see the iconic Treasury and the Monastery. Meanwhile, in Machu Picchu, Peru, trek through the breathtaking Andean landscapes to reach the ancient ruins and witness the sunrise over this incredible UNESCO World Heritage site.

For a 4-week adventure, the world is your oyster. Explore the bustling streets of Bangkok, Thailand, and visit the magnificent temples and palaces, or embark on a journey to witness the awe-inspiring beauty of Guilin, China, with its picturesque limestone karsts and serene rivers. Take in the stunning natural beauty of Banff National Park in Canada, and experience the captivating charm of Cesky Krumlov, Czech Republic, with its beautifully preserved medieval architecture. Whatever the duration of your visit, there is a wealth of experiences and sights to enjoy in these amazing destinations.

Note: The destinations mentioned are high in number, and it's not feasible to cover all of them in a 4-week duration. It's recommended to narrow down the destinations based on personal preferences and available time.