You can check the whether in Seoul and Jeju and thus you can plan your itinerary properly. Or at least you can do minor adjustment kan? You must choose the outdoor activities like Seongsan during sunny day. During rainy day, maybe you choose indoor activity like Museum.
2) Qibla Compass
No need to explain I guess, you must know the usage hehe
3) Subway Navigation
You really need to know which metro line that you suppose to take. Korea's metro lines is quite complicated but with this apps it really help us.
4) Google Translate
I used this app during negotiation at the market. hehe. Just type " discount please", then it will pronounce the sentence. It can translate english to korean and vice versa.
However, you must know basic korean language like greetings " Hello", "Bye", " Thank You" " Where is toilet". rugi la pergi korea tatau langsung bahasa sinun. You can easily build rapport by knowing little korean language hehe