The Banpodaegyo Bridge Moonlight Rainbow Fountain is an interesting 20-minute show at night in Seoul between April and October. There are 3 shows from 20:00 to 21:00 during the weekdays in April, June, September and October. The show begins earlier at 19:30 at the weekends, holidays and in the summer (July and August).
If you come to the area early, check out the Some Evit on the other side of the bridge which is an artificial island with multimedia gallery and coffee shops. The sunset is really gorgeous then you may walk across to see the Moonlight Rainbow Fountain performance.