Certainly! Exploring Denmark's charming cities is a fantastic plan. Here are some tips to ensure a smooth trip to Copenhagen, Billund, and Aalborg:

Best Time to Visit:
The best times to visit Denmark are in the spring (April to June) or autumn (September and October) when the weather is pleasant, and the crowds are smaller. The summer months (June to August) can also be lovely, but be prepared for larger crowds at popular sightseeing locations.

Route and Accessibility:
From abroad, one of the best ways to enter Denmark is through its capital, Copenhagen. From Copenhagen, you can take a train or drive to Billund and then to Aalborg. Denmark's public transport is efficient and punctual, making travel within the country easy.

Altitude and Acclimatization:
Denmark is a flat country, so you do not need to worry about altitude sickness or acclimatization.

Accommodation and Booking:
It is recommended to book accommodations in advance - you'll find options ranging from budget hostels, mid-range hotels, to luxury establishments. If you are traveling in peak season or planning to stay near popular tourist attractions, early booking is particularly important.

Clothing and Essentials:
During warmer months, comfortable clothes will work fine. But always have a jacket or sweater on hand as it can get chilly, especially in the evenings. For colder seasons, pack warm clothing and layers. Don't forget comfortable walking shoes, sunglasses, sunscreen, and an umbrella or raincoat (just in case).

Health and Safety:
Denmark is a safe country with a low crime rate. However, take basic precautions like keeping your belongings secure. If you're planning a lot of walking or cycling, make sure you have suitable travel insurance to cover any unexpected accidents.

Local Etiquette and Respect:
Danish people are courteous and respect personal space. Keep your voice down when in public places and respect the rules of the places you visit. Denmark is also a very eco-friendly country, so ensure you're mindful of your environmental impact during your visit.

Remember, the Danish truly value relaxing and enjoying their time, so take a page from their book and don't rush - soak in the scenery and enjoy your time!