Absolutely! Here are some tips for your trip to Seoul, Incheon, and Jeju:

Best Time to Visit:
For Seoul and Incheon, the best times to visit are during spring (April - June) and fall (September - November) when the weather is mild. As for Jeju, it is a year-round destination, but April - June is the best time.

Route and Accessibility:
Incheon is near Seoul and both cities are well connected by public transport. From Seoul, you can reach Jeju by a one-hour flight.

Altitude and Acclimatization:
Altitude and acclimatization are not much of a concern in Seoul, Incheon, or Jeju as they are at very comfortable altitudes.

Accommodation and Booking:
Accommodations in these cities range from budget to luxury. Make sure to book your accommodations in advance, especially during peak tourist seasons.

Clothing and Essentials:
Stay comfortable by dressing according to the season – layer in spring and fall, light clothes for summer, and warm attire for winter. Make sure to carry an umbrella since rain showers could happen anytime in Jeju.

Health and Safety:
South Korea is renowned for its safety. All the same, it's always a good idea to stay cautious. Do register with your embassy upon arrival, carry a first-aid kit, and have your medical insurance information with you.

Local Etiquette and Respect:
Koreans appreciate it if visitors respect their customs. Bowing is a common form of greeting. Also, remember to take off your shoes when entering someone's home or even some restaurants. Do try to learn a few basic Korean phrases, it can greatly enhance your travel experience. Dispose your waste properly and respect public properties and spaces.