Namdaemun Market, the most bustling place in Seoul. Food vendors fly through unnavigable aisles with food stacked on their heads, touts selling their wares and a constant stream of shoppers looking to spot the best bargain.
Three things you should buy in Namdaemun;
1. Korean Red Ginseng - POW, potent stuff that keeps you mentally and physically sharp.
2. Gochu Chocolate - A chocolate infused with the spicy Korean pepper, this stuff is affordable and perfect for taking on a plane.
3. Hanboks - Tourist can take home a handmade hanbok (something similar to a Kimono, only Korean). These can get pricey but a number of material and color combinations help reduce the price.
To get to Namdaemun, take Line 4 to Hoehyun and follow the crowds, this place is packed on the weekends so pick and choose how you would liek to experience it.
P.S. - keep an eye (and nose) open for yachae hotteok, a fried "pancake" stuffed with stir-fried vegetables and spiced glass noodles. The lines for these wrap around the corner so for 70 cents a pop by more than you can eat!