What is the best way to get smartphone access when traveling for a few days in South Korea? I am mostly interested in getting cheap data access (google maps...).
All major Korean cell-phone network providers (SKT,KT,LG) have their booth at the airport. They rent cell phones with daily charges.
If you can read Korean, look here.
You can also rent data sharing devices called egg (KT) or bridge (SKT). You can use them to get wifi wherever you can get cell service.
It is cheaper to buy a used cell phone, a pre-paid contract, and sell the phone back before you leave. It's quite common for regular (non-smart) phones. The procedure can take up to an hour, since you don't have a Korean ID and clerk will have to call/fax/etc some head office or support line to get your number registered to your passport.
Renting a phone is an option, but it is expensive.
Nowadays I think you can get a pre-parid microsim for your own GSM/LTE smartphone. All modern high-end smartphones are quite capable; coverage won't be as good as native's, but I think it will be good enough.
You can rent a so called "egg" from various website like www.taobao.com and it provides wifi hotspot, then you can use your cell phone connect to it and use apps like google map. I rented for less than $10 per day, and the data is unlimited. Speed is decent too.