Surely! Odense, Copenhagen, and Frederikshavn are gems of Danish tourism with rich history, culture, and natural beauty. Here’re a few tips to make your visit memorable:

Best Time to Visit:
The best time to visit these cities in Denmark is during late spring (April to June) and early autumn (September to October). These months offer pleasant weather with lesser crowds.

Route and Accessibility:
Copenhagen is well connected to major international cities. You can travel to Odense from Copenhagen by train in about an hour and a half. From Odense, Frederikshavn can be reached by a combination of train and bus, which takes slightly more than 4 hours.

Altitude and Acclimatization:
These cities are situated at sea level, reducing the challenges associated with altitude sickness. However, adjusting to the local weather and routine is always advisable.

Accommodation and Booking:
Both budget and luxury accommodations are available. Prefer choosing accommodation near the city center or close to public transport. It’s advisable to book in advance, especially during peak tourist season.

Clothing and Essentials:
Pack light clothes for summer, warm clothes for winter, and a waterproof jacket for sudden showers. Don’t forget your comfortable shoes for all the walking you'll be doing. Also, remember to carry essentials like sunscreen, universal adapters, etc.

Health and Safety:
Hygiene standards in Denmark are high, but it's always wise to have a well-stocked first aid kit handy. Stay cautious in crowded places. If you're planning to cycle, wearing a helmet is a must.

Local Etiquette and Respect:
Danish people are polite and reserved. Respect the local etiquette like obeying traffic rules, maintaining quiet in public transport, etc. Public drinking is accepted but drunkenness is frowned upon.

Lastly, relish Danish cuisine, embrace the culture, and enjoy the natural beauty. Denmark offers an array of attractions for every kind of traveler. Enjoy your trip!