One of the world's great cathedrals, the Cologne Cathedral is a marvel to view in person. The twin spires are the tallest on any cathedral in the world I think. It's an amazing piece of European and Germany history to see the in person. Make sure to walk all the way around for the full perspective.
Of course the main attraction if Cologne / Köln is the cathedral on the main square. But the city also offers other places that are very nice. For me the pedestrian street is super nice with plenty of shops and good cafés. It is very convenient for shipping tourism during the sale days.
Walking into the interior, under the majestic appearance, a delicate church greets you. The stained glass on the four walls with the characters of the Bible is particularly dazzling under the golden light. It is truly a world cultural heritage. When people walk into it, they are awe-inspiring, sitting in the corner and watching everything here, a silent The power surrounds me, solemn, and feels that time is still.
Come to the side door on the left side of the church and spend 4 euros to get to the spire of the highest point of Cologne Cathedral, overlooking the whole city of Cologne.