Here are some tips and recommendations about travelling in Korea! You may or may not follow my advice, but I believe this will make your planning and travelling easier!
Booking through 3rd party websites
I am sure some of us may book through 3rd party websites like booking.com or hotels.com. Always email the hotel/hostel with confirmations prior to your departure to ensure they have received your reservations.
Sign up as a member
When booking airfare or making a car rental reservation, sign up as a member first before purchasing your ticket as they offer discounts to members! I received 75% discount for my car rental in Jeju!
Renting of car
I would recommend renting a car in Jeju. There are no trains in Jeju and walking from bus stop to the attraction's entrance will be a very long distance (like 20 mins long). We rented a car for about $160+ for about 36 hours. It is affordable in Jeju, so if you have a driver with you, RENT A CAR! It really saved us a lot of time from waiting for buses and energy.
International Driving Licences
If you're renting a car in Jeju, please bring along your International Driving Licences. They will not rent it to you unless you have it. Making one in Singapore is easy, you just have to apply and pay for it online and send in the confirmation together with a passport sized photo.
Ask for English navigation
If you're renting a car, email the company to provide you with English navigation/GPS. It is based on first come first serve basis so you have to reserve it in advance. Most navigation comes complimentary with the car rental.
Bring a light jacket/windbreak along
If you happen to go during the shoulder period when Spring transits into Summer, please bring a light jacket/windbreaker along. Temperatures drop like crazy at night and you want to be prepared.
Most Koreans in Seoul are able to speak a little bit of English, so do not worry. We only faced difficulty in Busan and Jeju and so, we just ordered what we knew like Ramyeon or Jajangmeyon or point at pictures.
Medicine & Hydration
This really shouldn't be a tip, but a necessity for all travelers around the world. I fell terribly sick in Jeju after several nights of Korean BBQ (sore throat) and when I climbed Mt. Hallasan, the cold wind blew against my shirt (drenched in sweat) and I caught a terrible cold! I had very bad flu and fever that evening.
iPhone Subway App
For iPhone users, there's a Korean Subway App for you to download. It has a white train logo with blue background and it's just called Subway. It is really good because it helps you plan your travel route and tells you the fastest or cheapest route to take, what time the train arrives and what time you will arrive at your destination. It has subway maps of the major cities in Korea like Seoul, Busan, Daegu, etc.