Sunday is a hell in Seoul! The 24.5 million inhabitants all seem to be on the streets at once. If you want peace and quiet, visit Hwagye-sa.
It's an active Buddhist Temple and a home to the International Zen Centre, located at the base of Bukan mountain short 30 min. subway/bus trip from the heart of Seoul.
Go on Sunday and get a free vegan 'Monk Lunch' between 11.30-12.30 on the ground floor of the main building
After lunch there is an intro to beginners at 12.30 upstairs at the Zen Center. Worth the 30 minutes cross-legged sitting on a cushion :o)
Then to streach out a little, take an easy hike up to the mountain spring. Entrance just before the Temple's - follow the small path on the left, curving towards the right around the hill, (don't go straight on) It's a lovely walk.
I can't think of a more peaceful way to spend a Sunday in Seoul.
Subway line 4 (light Blue) to Suyu Station, exit 3,
take No.02 small green bus and get off at Hwagye-sa (Temple), cross the road and walk up the hill for 5 minutes or take taxi (under $3) from Suyu Station exit 3.