This app was mentioned in 9 comments, with an average of 2.56 upvotes
Spent just over 10 days last year..
Get a T-money card for traveling on trains and buses. You can pretty much get them at any convenience store. Top it up with 10000 won, then add on when it's going low. Each trip cost about 1000 won or so. Get the Subway Korea app, very convenient to get to know which station, train door, station exit, to get to where you want to be. This is, of course, within the Seoul metropolitan area. There's a KRpass for foreigners if you intend to go to other cities (Busan, Daegu, etc)
Most shops, except for the street carts type, accept visa/mastercard as payment, so you won't really need to bother too much about physical currency. Just sign away..
This is probably what I would do if I found myself in Korea again. This is something you may want to have.
I suggest making a trip to the iconic JYP building (assuming they haven't moved yet). Hongdae for hats and things, and ALL the busking. Especially at night, in Hongdae park (it's what I think it's called). Also, prepare for heat...or rain. Either one, umbrellas are very handy. Itaewon has nice non-Korean restaurants (I found an Indian buffet there that one time).
Or if you like pretty girls, Ewha Uni is a nice place to visit. <_<;;;
For the Subway, I use "Subway Korea" by Malang.
iOS: https://itunes.apple.com/gb/app/subway-korea/id325924444?mt=8
Android: https://play.google.com/store/apps/details?id=com.imagedrome.jihachul&hl=en
This is what I used for subway: https://play.google.com/store/apps/details?id=com.imagedrome.jihachul&hl=en
For maps you'll have to settle for naver maps, which I don't think has an English version.
I guess everyone's concept of what is cheap and expensive is kinda different. But as for me, I find cabs very cheap. You can travel fair amount of distance with 5 bucks.
If you are going to stay here for about one week, it won't be enough time to get used to the subway and bus systems, so it might seem overwhelming and confusing. So, you will absolutely need to get the Seoul Metro Subway App. I personally like to use these ones. Just click where you are and where you want to go and it will give you the most efficient way to get there. Android Apple I think google maps do a good job too, and it will include buses, but if you are new, it might be a little confusing. Honestly, cabs are cheap, so if you want to travel around and don't get lost, cabs might be a good option, too. There are also cabs with English speaking drivers.
With minimal Korean, you can definitely get around Korea. I think it's pretty accommodating for English speakers. As far as directions go, everything is written in English and Korean. If you want to ask someone for directions, 1 out of 5 people will definitely know how to explain with some basic Korean. I had a lot of foreign travel here, and I do not remember anyone having any difficulty getting around. As a matter of fact, they say that Koreans were very helpful.
As far as place that you want to go to. What kinda places do you like to check out? If you can narrow down to some of your preferences, I might be able to pinpoint more effectively.
For transfers, this information has forever been shown in the "Subway" app. https://play.google.com/store/apps/details?id=com.imagedrome.jihachul
I have used this one for years for the subway: https://play.google.com/store/apps/details?id=com.imagedrome.jihachul&hl=en
jihachul
https://play.google.com/store/apps/details?id=com.imagedrome.jihachul&hl=en
Did you even search play?