We mostly walked everywhere or took the subway. The subway has a total of 17 lines, all distinguished by different colours. There are plenty of signs in English which makes it easy to navigate your way around. Be sure to grab yourself an up to date subway map, they are very helpful when planning your trip.
Subway fares are based on the distance travelled, but the shortest ride costs 1,150KRW/$1.30AUD plus a 500KRW/$0.60AUD card deposit (but this is refundable if you return your card at the designated machines at the station). Single ride cards can be purchased from vending machines only. All vending machines accept coins and notes.