There's no Daiso inside the Han River parks. The closest to Yeouido Hangang Park is Daiso Yeouido (61 Yeouinaru-ro), about 700m away, so stop by on the way. It's open until 10 PM. The LED candle lights (3 pcs, ₩1,000) use no flame, so they're fine in the park.
Daiso Mall
Daiso MallDaiso Mall