Mitte District, where you'll find famous landmarks such as the Brandenburg Gate

1. Berlin
Brandenburg Gate
If you're only ever able to make a single trip to Germany, you'll want to spend at least a few days in Berlin. The country's capital is undoubtedly one of the most dynamic and vibrant cities in Europe, as popular for its superb dining experiences as it is for its shopping and entertainment.
Berlin is also considered one of Europe's top cultural destinations, home to numerous excellent art galleries and museums just waiting to be explored. Some of the best of which are located on Museum Island, a must-see destination that alone can take days to explore.
And after these attractions have closed, fun things to do at night include enjoying concerts by the renowned Berlin Philharmonic Orchestra, perhaps taking in a performance by Berlin Opera, or simply wandering the always-interesting avenues and boulevards in the Mitte District, where you'll find famous landmarks such as the Brandenburg Gate.
Other areas you'll want to explore include Charlottenburg. Generally a little quieter than the busy Mitte area, it's home to many pleasant parks and gardens including the Zoological Garden.

Cologne
Cologne at sunset
Cologne is another of those wonderful old German cities that has managed to preserve its past for modern generations to enjoy. Originally settled by the Romans, Cologne was for centuries an important religious center and a place of pilgrimage.
These days, the best places for visitors to go in Cologne are located in and around the Old Town, home to 12 classic old churches, including the famous Cologne Cathedral. Expect to spend hours exploring this magnificent structure and the surrounding former merchant homes, many of them now bustling with modern trade, including art galleries, boutique shops, and cafés.
Cologne's reputation as a center of trade and commerce has never diminished, with visitors drawn to its shopping areas and traditional marketplaces to purchase locally produced goods, from fashion and fragrances to food stuffs (you haven't lived until you sample some original Cologne-made chocolate, available in many locations around the Old Town). Other fun things to do in Cologne include exploring its ancient Roman remains, magnificent Baroque palaces, and countless museums and art galleries.
The city is also a popular stop in Germany for its location on the Rhine River, which slices through the city. The riverfront is not only a wonderful area to explore on foot, but also serves as a major embarkation point for Rhine river cruises that can last anywhere from a few hours to a few days.

Hagia Sophia Museum

If your first destination is Istanbul, don’t forget to visit the iconic Hagia Sophia Museum, one of the most popular tourist attractions in Turkey. A church built by Emperor Justinian in the 6th century, it was converted to a mosque after some 900 years. It is one of the most beautiful mosques in Istanbul and one of the best places to visit in Turkey.

Location: Sultan Ahmet, Ayasofya Meydanı, 34122 Fatih/İstanbul, Turkey
Things to do: explore the nearby places and learn about the history of the place
Entry Fee: 60 Turkish Lira per person. Free for children below the age of 12 years.
Timings: 9 am – 5 pm

Topkapı Palace

Built-in the 15th century by Mehmet the Conqueror, the palace remained both the symbolic and political home of the Ottoman Empire until 1853. This palace will take you back in the past with its decoration, treasure, and Haren, which is why it’s one of the most interesting places in Turkey for history geeks. All together makes this place spellbound and is a must-see for any tourist visiting Istanbul in Turkey. The palace was converted into a museum after the fall of the Ottoman Empire in 1924.
Location: Cankurtaran, 34122 Fatih/İstanbul, Turkey
Things to do: Learn about the history of the place
Timings: 9 Am – 6 pm
Entry Fee: 100 Turkish Lira per person. Free for children below the age of 12 years.