Nothing - but if you plan to picnic then a picnic mat and some food would be great!

BUS, SUBWAY, WALK

I've visited the Banpo Hangang Park many times, and it remains one of my favorite spots in Seoul to watch the sunset and enjoy the view. In warmer months, a lot of people picnic by the riverside, bringing their own mat and food. From April to October, there is the Banpo Bridge Moonlight Rainbow Fountain with showtimes 12:00, 19:30, 20:00, 20:30, 21:00. It's free and open for the public. Even when it gets really crowded, you can still get a good view of the fountain show because there is a lot of open space. The nearby Some Sevit (floating islands) and surrounding cityscape are also very beautiful at night.