South Korea Solo: Tangible Essentials

These are the things that were most useful (aside from money) during my 8-day trip to South Korea:
T-money

My very pink T-money card, the only
design available at the time.

T-money was my ticket to almost anywhere in South Korea—it's a prepaid transportation card that can be used for the subway, bus, and taxi. (It can also be used to pay for purchases at affiliated convenience stores, vending machines, lockers, etc.)
For me to have a T-money card, I had to have money to purchase one. A T-money card (no credits yet) would cost from KRW 2500 to KRW 4000 (for cards with special designs) and can be bought and added with credits (from KRW 1000 to KRW 90,000) at convenience stores or at subways stations.
Advantages:

Fare charged using the T-money card is KRW 100 cheaper than when paying with cash.
Eliminates the hassle of buying a ticket every time you take the subway.
No need to have the exact amount when boarding the bus, just tap the card upon boarding and before alighting.
The remaining balance, if less than KRW 20,000, can be refunded at convenience stores minus a service charge of KRW 500. Refund amount of more than KRW 20,000 can only be refunded at the T-money headquarters in Seoul, so be sure to put in just enough for your trip.

Tip: If you plan to be in South Korea for five days or more and need to take the subway and/or bus often, a T-money card would be advantageous in terms of savings. You can estimate the total subway expense by searching for the route and fare using Seoul Metropolitan Rapid Transit Corporation's Cyber Map (for Seoul) or Subway Korea app by Malang Studio Co Ltd (for Seoul) or Korea Subway app by Pengtai (for Busan, Daejeon, Daegu, Gwangju).
Smartphone
A late bloomer, I only got myself a decent smartphone less than a year ago. In years past, I never saw the need for one, and just went old school (paper maps!). Having a smartphone proved to be very useful for this trip...
Mobile Apps
...especially since I had to rely heavily on six apps:

Booking.com – This is where I found and booked all my accommodations: Kam Guesthouse in Seoul, Aroha Guesthouse in Seoul, and Sum Guesthouse Garosugil in Busan.
Visit Korea – I scoured the Visit Korea app for places to see in South Korea. In addition to photos and a description about the place, this app also provides important information like transportation directions, maps, operating hours, and admission fees.
Google Maps or Google My Maps – I used Google Maps/Google My Maps to find my way around. Being a map geek, I pinned places and created my own map using Google My Maps.
Weather – When in a country with four seasons, and when traveling during non-summer months, I need to check the temperature to know how many layers I should be wearing.
Subway Korea – This app made snaking my way around Seoul's extensive subway system (which extends to Incheon and to some places in the provinces of Gyeonggi and Gangwon) easier, faster, and smoother.
Korea Subway – Although this app also includes Seoul Metro, I used this app mainly for Daegu and Busan.

(I will bore you with details about these apps another time.)
WiFi Egg (Pocket WiFi)
But these apps wouldn't work without WiFi. Although there is free WiFi in most places, I decided to rent a pocket WiFi or a WiFi Egg as they call it in South Korea, in case I find myself lost in a no free WiFi zone.
A WiFi Egg (or mobile phone if you also need to make calls and send text messages) can be rented at the Roaming Center of Incheon International Airport (first floor, arrival hall, between gates 10 and 11) or at Gimhae International Airport (first floor, near gate 3). WiFi Egg rental rates are the same for all telecoms: Data KRW 5000/day + Device Rental KRW 3000/day + 10% tax.
These are the three telecoms (and their current promotions) to choose from:

Korea Telecom – Rental fee is free when reserved online.
LG U+ – For rentals of five days or more, rental fee is free on the 5th day onward.
SK Telecom – Rental fee is free until December 31, 2016.

Renting a WiFi Egg (or a mobile phone) requires a credit card and passport. The credit card should have enough credit limit to cover the data and rental fees for the rental duration AND the deposit for the device (KRW 200,000 for the WiFi Egg, more for mobile phone).
Tip: If you're planning to visit Seoul (flying in and out of Incheon) this year, avail of SK Telecom's free 5-day rental of Samsung Galaxy Note 5 with free data and voice calls.
Credit Card
The main reason I got myself a credit card was because I would be traveling alone and it could be used for (God forbid) emergencies. In the end, there were no emergencies (thank God), but I was able to use it to:

Buy airline tickets online and later on, buy add-on baggage and inflight meals.
Book accommodations online.
Rent a WiFi Egg.

Powerbank

South Korea uses
the type C Plug

A powerbank adds weight to my already heavy bag, but I needed it to resurrect a dead phone and/or a dead WiFi Egg.
Adapter
To resurrect dead gadgets when the powerbank dies too. Gadgets whose plugs are all other types except type C need an adapter to gather juice from South Korea's two-round-holed power outlets. (If you're coming from a 110V region, you might need a step up converter, too.)
Season-Appropriate Clothes
This and the Weather app go hand in hand. I visited in April and brought a jacket for the low temperatures and an umbrella for the spring showers.

Comfortable Walking Shoes

My feet did not thank me for walking every day for hours on end. But it did thank me for being in comfortable shoes.

What would I have done without all these things? I would have survived. But probably not without getting lost, missing my bus/subway stop, getting on the wrong subway line, dropping coins while fumbling for change and holding up the line in the bus, shivering in the cold for wearing too thin clothes, sweating in the heat in a thick jacket, soaking in the rain, and hobbling home with blistered feet.
South Korea on ThreeSouth Korea Countdown Itinerary and ExpensesSouth Korea Solo: Tangible Essentials (you're here!)(more soon)