I think it's not too late for me to review the best spring destination in Seoul. A couple place that is still have a cherry blossom.
Yeouido Cherry Blossom
Yeouido Cherry Blossom is the most popular place to visit when spring. But as for today the Yeouido Spring Flower Festival are already over but the cherry blossom are still there. Just take
How : Take Yeouinaru Station (Line No. 5) and take exit 1 or Exit 2.
Namsan Mountain's Cherry Blossom