There are different tariff rules depending on whether you are traveling within or outside the EU.
If you are planning a trip outside the European Union or the Nordic countries, it is a good idea to check beforehand if the countries you will visit require a visa of Danish citizens.
It is a requirement that you can verify your identity at the check-in point in the airport. It is often one's personal passport, which is the valid documentation. If you are abroad and do not have your passport, it is possible that other photo ID can be used for traveling back to your residential country.
If you find that your passport has expired before departure, you can immediately go to the citizen service and get the passport renewed for up to 1 year. This applies only if it has not previously been extended by 1 year, and if it is not more than 2 years ago the validity of the passport expired.