Hello! Sounds like you're gearing up for an exciting adventure to Denmark! Here are some travel tips for your visit to Odense, Copenhagen, and Frederikshavn:

Best Time to Visit:
The best time to visit these cities is from May to August when the weather is warmest. Spring (April to June) is particularly beautiful due to the blossoming flowers. However, do note that summer months (June to August) are the most crowded.

Route and Accessibility:
You can start your journey at Copenhagen, Denmark's capital, easily accessible via Copenhagen Airport (CPH). From there, the other cities are well connected by train or bus. It takes around 1.5-2 hours to reach Odense from Copenhagen by train, and then another 3-4 hours to Frederikshavn.

Accommodation and Booking:
All three cities offer a wide range of accommodations -- from luxury hotels to budget hostels and guesthouses. It's recommended to book your accommodations in advance, particularly if you're travelling during the peak tourism season.

Clothing and Essentials:
Pack lightweight clothing for the summer with an additional layer for cooler evenings. Carry a good quality rain jacket or an umbrella, as Denmark often has unexpected rain showers. Comfortable walking shoes are a must as you'll likely be doing a lot of walking or biking.

Health and Safety:
Denmark is generally a safe country with excellent healthcare facilities. Ensure you have travel insurance covering any medical emergencies. If you're an EU citizen, bring your European Health Insurance Card (EHIC).

Local Etiquette and Respect:
Danes generally appreciate politeness and punctuality. It's good to respect local customs and personal space. It's also common practice to say 'Tak' (Thank you) when provided a service.