Hello! It sounds like you're planning an exciting trip to Denmark, specifically to the beautiful cities of Copenhagen, Hillerød, and Helsingør. Here are some travel tips to help you with your journey.

Best Time to Visit:
The best time to visit Denmark is in the summer, from June to August. The weather is warmest during this period. However, Copenhagen can also be enjoyed in the shoulder season (May and September) when the weather is still pleasant and the crowd is less.

Route and Accessibility:
The Copenhagen Airport, Kastrup, is the main international gateway to Denmark. From the airport, you can take a train to Copenhagen Central Station. To visit Hillerød and Helsingør, you can take the E line train from Copenhagen Central Station.

Accommodation and Booking:
Accommodation options in Copenhagen range from luxury hotels to budget hostels. It's recommended to book your accommodation in advance, especially if you're visiting during the summer peak season. In Hillerød and Helsingør, you may find fewer hotels but there are still a number of good options available.

Clothing and Essentials:
Even though summer can be warm, it is advisable to carry a rain jacket or umbrella because rain showers can occur at any time. A comfortable pair of walking shoes is essential as these cities are best explored on foot.

Health and Safety:
Denmark is generally a safe country with low crime levels. It's always advisable to carry a basic first aid kit and your regular prescription medicines, if any. Make sure to have a valid travel insurance that covers emergency medical situations.

Local Etiquette and Respect:
Danes appreciate politeness and respect for personal space. It’s best to avoid talking loudly on public transportation. Also, remember to respect cycling lanes and always cross the road at designated crosswalks.