I'm travelling to Denmark with a Schengen visa type C single entry for 21 days.
Can I travel to Paris or Switzerland and back to Denmark with the same visa?
Travel Stack Exchange is a question and answer site for road warriors and seasoned travelers. It only takes a minute to sign up.Sign up to join this community
You can do it if you have said so in your visa application, or if it is a "minor" adjustment of your travel plans.
Your Schengen visa allows you to enter the Schengen area once and stay for 21 days (if your 21 days are the duration and not the validity period, read up on the difference). Within the Schengen area, you can cross internal borders as many times as you like.
But you should not give the impression that you lied in your application.