What is Reddit's opinion of Korea Subway Info : Metroid?

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1. take out USD in california and exchange it in itaewon. stay away from the airport exchanges as you will get a better deal at a mom and pop place. some ATMs accept non-korean cards but they can be a pain in the butt to find. the fees around what they are here.
2. make sure your phone is compatible with the korean carrier bands, and then find somewhere to buy a prepaid sim. you can survive on data alone. heck, most places in korea have free wifi if they don't have olleh/kt/skt wifi. you can use www.willmyphonework.net as a resource
3. depends where you're staying but the subways and buses typically run until midnight. idk if you are an android or ios user but these apps should come in handy for you as far as public transit goes: https://play.google.com/store/apps/details?id=com.whitecrow.metroid for subway, https://play.google.com/store/apps/details?id=net.daum.android.map for bus. you need to know a little hangeul to use daums maps effectively. if you are totally new to the language avoid the app and the bus system altogether if you can. i suggest getting a bus ticket from ICN to seoul because it will feel less rushed than all the transfers you might have to do via the subway. but then again, i had an incheon airport bus stop right near my apartment when i lived in korea so ymmv.
4. hostel accomodations are pretty inexpensive. look for hostels in itaewon/hongdae/myeongdong because they are in the middle of seoul and have a bunch of foreigner friendly touristy shit you might find interesting. you can always look for a good deal on a place to sleep at airbnb. i used airbnb every time i went to japan and it was a great experience, so i assume it should be the same in a country that's an hour flight away. some airbnb places also have washer and dryer so you dont have to spend extra money at a laundromat.

Take the subway. Download an app called MetroidHD (https://play.google.com/store/apps/details?id=com.whitecrow.metroid&hl=en ), select Incheon Airport then Myeongdong. Follow the path. It is easy as pie and all stations have english instructions. Id also suggest going to Itaewon if you are english speaking. Has tons of clubs and eateries as well. From Incheon airport follow the path to the subway by turning right out of the gates and follow the icons to the subway (youll need to stop at the tickets booth and buy a subway pass, ~15,000Won will be enough). Will take you about an hour to either Myeongdong or Itaewon each way.

/edit six hours is plenty to enjoy either location.

One of my favorite places https://en.wikipedia.org/wiki/Gwangjang_Market. Find the bindaetteok alley (https://en.wikipedia.org/wiki/Bindaetteok) find this shop 순희네 빈대떡 (http://m.blog.naver.com/teemoticon/220699587500) it is the best. Also, Tim Burton was there once.

Public transportation is pretty easy for foreigners. The subway is easiest. Lots of english signs and info available. Buses are a little harder because not as much english signage, but the stops are announced in english on most lines. Not so true if you go to smaller cities. Get a T-Money card (https://english.visitkorea.or.kr/enu/TRP/TP_ENG_8_1_1.jsp) load it up with money in a subway station and you can go anywhere. It is accepted at all subways, buses, most taxis (unless they are sketchy) and you can even use the card to make purchases at a lot of convenience stores.

This app will let you check out subway routes, times, and other info in English https://play.google.com/store/apps/details?id=com.whitecrow.metroid

If you can read Korean, Kakaomap is the way to go https://play.google.com/store/apps/details?id=net.daum.android.map

And if you can read Korean there are some other apps for booking buses and trains to other cities. My Korean isn't great, but I make use of a lot of these. Especially Kakoamap, because I can use it for door to door directions.