In Hongdae, go to Daiso Hongdae 2 (182 Yanghwa-ro) or Hongik Univ.

Entrance (25 Hongik-ro), both 10am–10pm. The birthday tassel garland is ₩1,000; 생일 축하 means Happy Birthday. Blow up the balloons yourself, and check if your hotel lets you stick things on the wall. Garlands and balloons are light, so pack them home as a keepsake.