Nyhavn is part of the original Copenhagen Harbor all the way back to the founding of Haven, as Copenhagen was then called, in the 12th century.
Today, old sailing vessels and the Harbor Canal Tour boats are the only boats observed, contributing to the unique atmosphere in Nyhavn.
When in Nyhavn, the Danes really get into the typical Danish term "Hygge" - we relax and forget all about the worries of our life. Now it's time to have a good time in the company of friends. That's all that matter.
It's customary to walk up and down the quays once or twice and take in the atmosphere and greet friends that might also be there.
It's amazing how many people you know that you meet in Nyhavn when the weather is good - it's the place Danes automatically focus at when the weather is right.
Relaxation is the key concept here. Having a bottle of beer within reach is not too bad an idea, either. In Denmark, contrary to eg. other Nordic countries, it's not only allowed to sit and drink in the street, it's a common practice by all, regardless of social standing or age. It is considered part of the concept of "Hygge" - have fun, relax, enjoy the weather, get into a good mood.