Hello! That's a fantastic choice of cities to visit in Denmark. Here are some tips for your trip:

Best Time to Visit:
The best time to visit Denmark is during the months of May to August when the weather is warmest. However, should you prefer lesser crowds, the shoulder seasons of April to May and September to October are also pleasant.

Route and Accessibility:
You can easily access Copenhagen, the capital, by direct international flights. Then domestic flights or trains can connect you to Billund and Aalborg effectively. Denmark's public transportation network is very efficient.

Accommodation and Booking:
There are numerous hotels in all price ranges in Copenhagen, Billund, and Aalborg. For a more authentic experience, consider booking a traditional Danish bed and breakfast. It is recommended to book in advance especially during peak travel season.

Clothing and Essentials:
Pack both warm and cool clothing as Danish weather can be unpredictable. Rain gear and comfortable walking shoes are a must. Also, don’t forget your travel adapter for European sockets.

Health and Safety:
Denmark is generally a safe country with a low crime rate. It's advisable to have travel insurance for any unexpected medical emergencies. Don't forget to keep yourself updated with vaccinations like MMR, Diphtheria-Tetanus-Pertussis, Varicella, Polio, and your yearly flu shot.

Local Etiquette and Respect:
Danes appreciate punctuality and politeness. When visiting someone's home, it is customary to bring a small gift. Also, refrain from littering as Danes are very environment-conscious.