Probably all the big shopping malls , famous tourist attractions like Insadong, myungdong or busy people..
but here's a very interesting place in Seoul.
That's Hwanghak-dong Flea Market..
It feels so different from any other places in Seoul..
Hwanghak dong flea market (황학동 벼룩시장) is an extensive used-goods market with over 500 shops.
In the past, the Hwanghak-dong flea market was the nation’s trade center for antique goods, but now, the market offers a variety of products including cameras, refrigerators, washing machines, TVs, watches, stationary, imported goods, compasses, antique cameras, perfume, old furniture, and more.
Dongmyo station (line1,6) exit 3->go in the opposite direction of the exit and turn right at the first alley then you can see the flea market..
If you exit out from the station, you can see a lot of vendors selling different stuff..
There's actually no sign that says "Flea market" anywhere near the station but if you go to the place you'll surely know where the flea market is..
Plus, you can also try to eat some Jeon(전-Korean pancake)..
and one of the good things is that the place is near Cheonggye stream(청계천).
Why don't you try to go there and experience the different part of Seoul?