Every year, from April to October, at Ttukseom Han riverside area, there are open flea markets on weekends.
Locals register the website and participate to sell their used things. As all flea markets are so, you can find some nice stuff with very reasonable price.
But what I also like here, there are some local stall food shops. They are from country side not from Seoul. So I have the chance to taste some special delicacy. Next to the market, there is special zone where you can build a tent to take a rest. There are also some water leisure agencies too. If you want to order some delivery food, like pizza, fried chicken, jjajang myeon, you can call and pick up at the delivery pick up point.
Nice event to spend time with family, kids, friends and couples.