Although you choose the best season to travel, you cannot control the weather. It would always be wise to make a plan B for a rainy day. However, visiting indoor attractions for all day can be head aching. Unless you choose to travel during a storm, there are places you can enjoy a nice walk with fresh air.
1. Bijarim Forest (http://english.jeju.go.kr/index.php/contents/travel/attractions/forests?act=view&seq=50100) Located in Gujwa-eup, about 10-15 min. driving from Sehwa beach, the forest walk will give you a wonderful refreshment. It takes 1 hour to 1.5 hour to finish the walk. Entrance fee is about KRW1500 for an adult. I would highly recommend to anyone who loves greenery. Bijarim forest is also recommended in a hot summer day.
2. Namwon Keuneong Beach walk: It is located at Kumho Jeju Resort. Go to Kumho resort(park your car at the resort if you are driving) and take a walk to the beach in front of the resort. It is also Olle trail route 5. The path is covered with trees and plants. It is also nice to visit in a hot sunny day.
3. Saryeoni forest walk: The entrance starts at Bijarimro(Road No.1112). This is beloved by hard-core Jeju lovers. It takes 3 hours to 6 hours to complete depending on the trail you choose. Don’t worry. You don’t need to complete the whole trail but just take a short walk and enjoy the forest. No toilets, no rubbish bins along the path so be prepared and bring back all your garbage. There is no public or private car parks so you might need to park your car along the road side. Try to avoid during sunny weekends. Parking can be a head-ache.