hi, i just came back from seoul yesterday...
must try their streetstalls, lots of them everywhere. try their korean pork/beef bbq as well as eateries. the food is cheap.
buy skincare, got faceshop, skinfood, etude house.....prices much much lower than sg price. and they give u lots of samples
i also went to Lotte Mart to shop for kimchi, seaweed and korean snacks.
my first trip too and i really enjoyed it.
u can try bargaining at the namdaemun & dongdaemun markets but dont expect to get much discount. maybe at most 20%. be friendly else they show u face...
the subway is the most convenient for getting around. its v easy to travel in seoul. enjoy ur trip!