The ports in northern Europe can be reached by sea. The ports like Sweden, Norway, Poland, Germany, UK and Iceland can be approached from Denmark through sea. Therefore, you can reach Denmark from these countries the same way as well.
Denmark has international airport in Copenhagen. It is connected to all major destinations throughout the world. There are airports also located at Esbjerg, Aalborg and Billund, which receive the flights from UK and Scandinavia.
Buses and cars run between Germany and Denmark. You can find out the the bus route from or to Denmark, and have convenient ride. If you have relatives in the country, or you can hire a car, then movement between Germany and Denmark will be completely under your control.